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| United States Patent |
6,504,636
|
|
Seto
, et al.
|
January 7, 2003
|
Optical communication system
Abstract
An optical communication system for optically transmitting transmission
data from a transmitting station to a transmitting device includes an
adder for adding an intermediate frequency subcarrier signal modulated
with data to be transmitted to a pilot carrier signal as a sinusoidal
wave, and an electro-optical converter for electro-optically converting
the above sum signal to an optical signal by directly modulating a
semiconductor laser element having a resonant frequency fr in accordance
with the sum signal and transmitting the signal to an optical fiber for a
down link. The frequency f.sub.IF of the intermediate frequency subcarrier
signal and the frequency f.sub.LO of the pilot carrier signal satisfy
f.sub.LO -f.sub.IF.gtoreq.1 [GHz],
2.times.f.sub.IF <f.sub.LO <(2/3).times.fr,
f.sub.IF <1 [GHz],
and
2 [GHz]<f.sub.LO.
| Inventors:
|
Seto; Ichiro (Fuchu, JP);
Tomioka; Tazuko (Tokyo, JP);
Ohshima; Shigeru (Yokohama, JP)
|
| Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
| Appl. No.:
|
329574 |
| Filed:
|
June 10, 1999 |
Foreign Application Priority Data
| Jun 11, 1998[JP] | 10-163561 |
| Oct 30, 1998[JP] | 10-309981 |
| Current U.S. Class: |
398/91; 398/32; 398/163; 398/204; 455/522 |
| Intern'l Class: |
H04B 010/12; H04B 007/00; H04J 014/02 |
| Field of Search: |
359/173,124,126,133,161-162,166,188
|
References Cited [Referenced By]
U.S. Patent Documents
| 3717814 | Feb., 1973 | Gans | 455/504.
|
| 5339184 | Aug., 1994 | Tang | 359/124.
|
| 5475707 | Dec., 1995 | Ficarra et al. | 375/208.
|
| 5615034 | Mar., 1997 | Hori | 359/110.
|
| Foreign Patent Documents |
| 6-98365 | Apr., 1994 | JP.
| |
| 6-164427 | Jun., 1994 | JP.
| |
| 6-350537 | Dec., 1994 | JP.
| |
| 8-316908 | Nov., 1996 | JP.
| |
Primary Examiner: Chan; Jason
Assistant Examiner: Payne; David C.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Claims
What is claimed is:
1. An optical communication system comprising:
an adder for adding an intermediate frequency subcarrier signal modulated
with data to be transmitted and a pilot carrier signal; and
an electro-optical converter for directly modulating a semiconductor laser
element having a resonant frequency fr using a signal output from said
adder and sending an optical signal through an optical fiber,
wherein a frequency f.sub.IF of the intermediate frequency subcarrier
signal and a frequency f.sub.LO of the pilot carrier signal satisfy:
f.sub.LO -f.sub.IF.gtoreq.1[GHz]
and
2.times.f.sub.IF <f.sub.LO <(2/3).times.fr.
2. The system according to claim 1, wherein
2[GHz]<f.sub.LO.
3. The system according to claim 1, further comprising:
a transmitting station having said adder and said electro-optical
converter, and
a transmitting device, connected to said transmitting station through said
optical fiber, comprising:
an opto-electrical converter for receiving the optical signal transmitted
through said optical fiber;
a separator for extracting the intermediate frequency subcarrier signal and
the pilot carrier signal from a received signal output from said
opto-electrical converter;
a frequency converter for converting the frequency of the intermediate
frequency subcarrier signal using the pilot carrier signal in accordance
with an output from said separator to obtain a radio frequency signal; and
a transmitter for transmitting the radio frequency signal obtained by said
frequency converter.
4. The system according to claim 1, further comprising:
a transmitting station having said adder and said electro-optical
converter; and
a transmitting device, connected to said transmitting station through said
optical fiber, comprising:
an opto-electrical converter for receiving the optical signal transmitted
through said optical fiber;
a separator for extracting the intermediate frequency subcarrier signal and
the pilot carrier signal from a received signal output from said
opto-electrical converter;
a multiplier for multiplying the frequency of the pilot carrier signal
output from said separator;
a frequency converter for converting the frequency of the intermediate
frequency subcarrier signal output from said separator using the pilot
carrier signal output from said multiplier to obtain a radio frequency
signal; and
a transmitter for transmitting the radio frequency signal obtained by said
frequency converter.
5. The system according to claim 1, further comprising:
a transmitting/receiving station having said adder, said electro-optical
converter, an opto-electrical converter, and a demodulator; and
a transmitting/receiving device, connected to said transmitting/receiving
station through said optical fiber, comprising:
an opto-electrical converter for receiving the optical signal transmitted
through said optical fiber from the electro-optical converter of the
transmitting/receiving station;
a separator for extracting the intermediate frequency subcarrier signal and
the pilot carrier signal from a received signal output from said
opto-electrical converter;
a transmission system frequency converter for converting the frequency of
the intermediate frequency subcarrier signal using the pilot carrier
signal in accordance with an output from said separator to obtain a first
radio frequency signal;
a transmitter for transmitting the first radio frequency signal obtained by
said transmission system frequency converter;
a receiver for receiving a second radio frequency signal;
a reception system frequency converter for converting a frequency of the
received second radio frequency signal output from said receiver using the
pilot carrier signal output from said separator to obtain the intermediate
frequency subcarrier signal; and
an electro-optical converter for converting the intermediate frequency
subcarrier signal output from said reception system frequency converter
into an optical signal and transmitting the optical signal through an
optical fiber for the opto-electrical converter of the
transmitting/receiving station.
6. The system according to claim 1, further comprising:
a transmitting/receiving station having said adder, said electro-optical
converter, an opto-electrical converter, and a demodulator; and
a transmitting/receiving device, connected to said transmitting/receiving
station through said optical fiber, comprising:
an opto-electrical converter for receiving the optical signal transmitted
through said optical fiber from the electro-optical converter of the
transmitting/receiving station;
a separator for extracting the intermediate frequency subcarrier signal and
the pilot carrier signal from a received signal output from said
opto-electrical converter;
a multiplier for multiplying the frequency of the pilot carrier signal
output from said separator;
a transmission system frequency converter for converting the frequency of
the intermediate frequency subcarrier signal output from said separator
using the pilot carrier signal output from said multiplier to obtain a
first radio frequency signal;
a transmitter for transmitting the first radio frequency signal obtained by
said transmission system frequency converter;
a receiver for receiving a second radio frequency signal;
a reception system frequency converter for converting a frequency of the
received second radio frequency signal output from said receiver using the
pilot carrier signal output from said multiplier to obtain the
intermediate frequency subcarrier signal; and
an electro-optical converter for converting the intermediate frequency
subcarrier signal output from said reception system frequency converter
into an optical signal and transmitting the optical signal through an
optical fiber for the opto-electrical converter of the
transmitting/receiving station.
7. The system according to claim 3, wherein said separator comprises a
bandpass filter.
8. The system according to claim 3, wherein said separator comprises a
low-pass filter for extracting the intermediate frequency subcarrier
signal and a high-pass filter for extracting the pilot carrier signal.
9. The system according to claim 3, wherein
said transmitting device comprises a plurality of antennas, and
said transmitting station directly modulates said semiconductor laser
element by a sum signal of a plurality of intermediate frequency
subcarrier signals having different frequencies corresponding to said
plurality of antennas and pilot carrier signals.
10. The system according to claim 3, wherein
said transmitting station is connected to an optical divider through said
optical fiber, and
a plurality of transmitting devices are connected to said optical divider
through optical fibers.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an optical communication system for
transmitting a high-frequency analog signal such as a radio signal via an
optical fiber.
This application is based on Japanese Patent Application No. 10-163561,
filed Jun. 11, 1998 and Japanese Patent Application No. 10-309981, filed
Oct. 30, 1998, the contents of which are incorporated herein by reference.
Along with recent development of mobile communication, expansion of radio
communication service areas is required. To effectively utilize radio wave
frequency resources and reduce cost of base station equipment, a scheme in
which individual radio zones (cells) are made small, and instead, a number
of radio zones are arranged at a high density has received a great deal of
attention. This is called a picocell radio zone. To realize the picocell
radio zone, a radio communication base station arrangement in which
transmitting/receiving devices and transmitting/receiving stations are
connected through optical fibers has been examined.
More specifically, a radio base station has transmitting/receiving stations
and transmitting/receiving devices. A plurality of transmitting/receiving
devices are prepared for one transmitting/receiving station. The output
power from each transmitting/receiving device is made small for the
picocell radio zones. The transmitting/receiving devices and the
transmitting/receiving station are connected through optical fibers. The
transmitting/receiving devices transmit signals received from a common
transmitting/receiving station to subscribers and transmit signals
received from subscribers to a common transmitting/receiving station. The
output from each transmitting/receiving device is made small to reduce
cost.
A transmitting/receiving device is mainly formed from an antenna section
and placed in each cell. A transmitting/receiving station has a modem and
a controller corresponding to the plurality of transmitting/receiving
devices in the cells. Therefore, the transmitting/receiving station is
also called a central control terminal station. An analog radio signal is
optically transmitted through an optical fiber between the
transmitting/receiving device and transmitting/receiving station. With
this arrangement, each transmitting/receiving device can be made simple,
compact, and low-cost, and one radio communication base station can
provide a number of cells.
In this arrangement, the basic arrangement of a transmitting/receiving
device includes only an antenna, and opto-electric and electro-optic
conversion devices and does not depend on the data rate or modulation
scheme of a radio signal. Therefore, even when the radio transmission
scheme is changed, replacement of the transmitting/receiving device or
change in constituent elements of the transmitting/receiving devices is
unnecessary.
For the above optical analog transmission, an electro-optical converter
(E/O converter) is required to convert an electrical signal into an
optical signal. At the E/O converter, light intensity of a semiconductor
laser element is modulated with a radio frequency signal. As the
modulation scheme, a scheme of directly modulating a semiconductor laser
element or a scheme using an external optical modulator is employed.
Advantages and disadvantages of these two schemes will be compared. In
terms of modulation distortion characteristics, device scale, and device
cost, the scheme of directly modulating a laser element is more
advantageous.
However, the trend of technology obviously indicates carrier frequency
shift to a higher frequency band, e.g., shift to the 2- to 5-[GHz] band as
the capacity of a radio frequency signal increases. However, in a
distributed feedback laser element (DFB-LD) as a representative laser
element, the modulation frequency range with a relatively small modulation
distortion is as low as 2 to 3 [GHz]. Therefore, direct modulation of a
laser element using a radio frequency signal is becoming difficult.
As disclosed in, e.g., Japanese Patent Publication (KOKAI) No. 6-164427, a
scheme (subcarrier transmission) of superposing an intermediate frequency
subcarrier signal f.sub.IF modulated by a data signal on a pilot carrier
signal f.sub.LO as a sinusoidal wave and optically transmitting the
superposed analog signal from a transmitting/receiving station to a
transmitting/receiving device has been proposed.
In the transmission scheme proposed in this prior art, the intermediate
frequency subcarrier signal f.sub.IF is frequency-converted (up-converted)
by a multiplied signal obtained by multiplying the received pilot carrier
signal f.sub.LO on the transmitting/receiving device side, thereby
obtaining a radio frequency signal. The laser element is used in a low
frequency band with excellent modulation distortion characteristics, and
the pilot carrier signal f.sub.LO is superposed on a frequency close to
the intermediate frequency subcarrier signal f.sub.IF.
According to an embodiment described in the above prior art, a pilot
carrier signal f.sub.LO having a frequency of 300 [MHz] is superposed near
an intermediate frequency subcarrier signal f.sub.IF in the 200-[MHZ]
band, as shown in FIG. 1. In this scheme, on the transmitting device side,
to ensure the noise characteristics of the radio frequency signal and
increase the frequency stability, the CNRs (Carrier-to-Noise Ratios) of
the received intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO must be high. That is, the noise level must be
low.
However, in the frequency band near the pilot carrier signal f.sub.LO, the
RIN (Relative Intensity Noise) increases. Therefore, when the pilot
carrier signal f.sub.LO is arranged near the frequency band of the
intermediate frequency subcarrier signal f.sub.IF, as in the prior art,
the CNR decreases.
FIG. 2 shows the result of an experiment conducted by the present
inventors. When the intermediate frequency subcarrier signal f.sub.IF is
set at 500 [MHz] and the pilot carrier signal f.sub.LO is set at 550
[MHz], the RIN characteristics largely degrade in accordance with the
optical modulation index of the pilot carrier signal f.sub.LO and, more
particularly, at an optical modulation index of 15 [%] or more, as shown
in FIG. 2. Therefore, the communication quality of a radio frequency
signal greatly degrades.
Especially, when the optical modulation index of the pilot carrier signal
f.sub.LO increases, degradation in RIN becomes conspicuous. Hence, a radio
frequency signal generated by frequency-converting the intermediate
frequency subcarrier signal f.sub.IF using the pilot carrier signal
f.sub.LO contains a number of noise components and therefore has poor
transmission characteristics. When a radio frequency signal containing a
number of noise components is transmitted, the noise components adversely
affect other radio frequency signals to impede radio communication.
Solutions to this problem are required.
To cope with a shortage in channels due to the recent increase in number of
subscribers or an increase in transmission rate, extensive studies have
been made for radio communication using a frequency band higher than the
conventional frequency band, e.g., millimeter waves or submillimeter
waves. For this system as well, an arrangement for connecting
transmitting/receiving devices and transmitting/receiving stations through
optical fibers has been examined.
As a connection form using optical fibers, a PON (Passive Optical Network)
is used. In the PON, as shown in FIGS. 3 and 4, a transmitting/receiving
station 1 and a plurality of transmitting/receiving devices 2 are
connected through optical fibers 4 in which a passive optical divider 3 is
inserted. An optical signal transmitted from the transmitting/receiving
station 1 to the optical fiber 4 is divided by the optical divider 3
inserted into the optical fiber 4, and distributed to the
transmitting/receiving devices 2.
In the PON, a passive optical divider 3 is inserted midway along optical
fibers 4 to accommodate the plurality of transmitting/receiving devices 2.
Hence, the optical transmission/reception device of the
transmitting/receiving station 1 and optical fibers 4 can be shared, and
accordingly, the equipment can be made compact.
In the PON, an optical signal transmitted from the transmitting/receiving
station 1 is divided, so the same signal reaches the plurality of
transmitting/receiving devices 2. There is no problem when radio signals
transmitted from the plurality of transmitting/receiving devices are
completely equal. However, different transmitting/receiving devices 2
normally transmit different radio signals.
Conventionally, as shown in the spectrum arrangement in FIG. 5, an optical
signal to be transmitted from the transmitting/receiving station to the
transmitting/receiving device is frequency-multiplexed while changing the
frequency of the intermediate frequency subcarrier signal f.sub.IF
corresponding with each transmitting/receiving devices and sent
(subcarrier multiplex transmission scheme). In this case, each
transmitting/receiving device receives the optical signal, extracts a
component to be transmitted from the self station, converts the component
into a radio signal frequency, and transmits the signal from the antenna.
In the example shown in FIG. 5, the frequencies of the intermediate
frequency signal are assigned at an appropriate interval and
frequency-multiplexed: for example, a signal f.sub.IF1 to a
transmitting/receiving device 2-1 is assigned near 100 [MHz], a signal
f.sub.IF2 to a transmitting/receiving device 2-2 is assigned near 200
[MHz], and a signal f.sub.IF3 to a transmitting/receiving device 2-3 is
assigned near 300 [MHz]. Therefore, if radio signals sent from the
transmitting/receiving devices 2-1, . . . , 2-3 are in the 2 [GHz] band,
the transmitting/receiving device 2-1 must up-convert the signal F.sub.IF1
by 1.9 [GHz], the transmitting/receiving device 2-2 must up-convert the
signal f.sub.IF2 by 1.8 [GHz], and the transmitting/receiving device 2-3
must up-convert the signal f.sub.IF3 by 1.7 [GHz].
For a conventional radio system using optical subcarrier transmission, a
method has been proposed in which not only the intermediate frequency
subcarrier signal f.sub.IF but also the pilot carrier signal f.sub.LO as a
signal for maintaining the frequency stability of the radio wave
transmitted from the transmitting/receiving devices is transmitted, and
each transmitting/receiving device frequency-converts (up-converts) the
intermediate frequency subcarrier signal f.sub.IF using the pilot carrier
signal f.sub.LO, as shown in FIG. 1.
As a consequence, when the frequencies of intermediate frequency subcarrier
signals for the individual transmitting/receiving devices are different,
as shown in FIG. 5, pilot carrier signals f.sub.LO for frequency
conversion must be prepared for the respective intermediate frequency
subcarrier signals f.sub.IF. Pilot carrier signals f.sub.LO corresponding
to the number of intermediate frequency subcarrier signals multiplexed
must be sent. These signals are multiplexed and sent in optical
transmission.
As a result, the total number of signals including the pilot carrier signal
f.sub.LO increases. Since the optical modulation index of the intermediate
frequency subcarrier signals in optical transmission is shared by the
pilot carrier signals f.sub.LO, the optical modulation index decreases to
degrade the transmission quality.
In the radio system, when a plurality of radio base stations
(transmitting/receiving devices) provide the same service, frequencies
slightly different from each other in the same frequency band are
sometimes used to prevent interference between signals from adjacent base
stations.
For example, frequencies are separated at an interval of 100 [kHz] in the 2
[GHz] band. When such transmitting/receiving devices are accommodated
through one fiber, a system in which subcarrier signals with different
frequencies are multiplexed in the radio frequency band while only one
pilot carrier signal is transmitted can be constructed.
In this case, however, each transmitting/receiving device 2 that has
received the optical signal from the transmitting/receiving station 1 must
select a signal to be used in the self station from signals arranged at an
interval as small as 100 [kHz]. For this purpose, a very steep filter with
high frequency stability is required, resulting in an increase in cost. In
a radio system using a radio scheme other than frequency multiplexing,
e.g., CDMA, signals transmitted from transmitting/receiving devices are in
the same frequency band. Therefore, the method of transmitting only one
pilot carrier signal f.sub.LO using a steep filter cannot be used.
To simplify the arrangement, an intermediate frequency signal to be used in
the self station must be separated from intermediate frequency signals,
which are multiplexed as subcarriers, using a simple filter, as described
above. For this purpose, subcarriers are preferably multiplexed at a large
frequency interval.
However, to do this, a plurality of pilot carrier signals f.sub.LO
corresponding to the number of the intermediate signals f.sub.IF are
necessary. To stabilize transmission quality, the optical modulation index
should not be decreased. Sending more signals including a plurality of
pilot carrier signals f.sub.LO means increasing the optical modulation
index for the total signals. The amount of RIN corresponds to the optical
modulation index. There is the effect of interference modulation as one of
the others noise decreasing transmission quality. The effect of the
interference modulation also corresponds to the number of signals and the
optical modulation index. The plurality of pilot carriers inevitable
degrades the data transmission quality.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an optical
communication system which can generate a radio frequency signal excellent
in noise characteristics without decreasing the CNR of an intermediate
frequency subcarrier signal f.sub.IF when a laser beam is directly
modulated using a signal obtained by synthesizing the intermediate
frequency subcarrier signal f.sub.IF and a pilot carrier signal f.sub.LO
to optically transmit the analog intermediate frequency subcarrier signal
f.sub.IF and pilot carrier signal f.sub.LO with a large optical modulation
index.
It is a second object of the present invention to provide an inexpensive
and simple optical communication system which can reduce the number of
pilot carrier signals f.sub.LO to be sent from a transmitting/receiving
station to transmitting/receiving devices, that are necessary for
frequency conversion, multiplex subcarriers at a large frequency interval,
and separate the intermediate frequency subcarrier signals using a simple
filter.
An optical communication system according to the present invention, which
multiplexes a subcarrier signal and a pilot carrier signal and optically
transmits the multiplexed signal from a transmitting/receiving station to
a transmitting/receiving device has the following arrangement.
The frequency band of a pilot carrier signal f.sub.LO and that of a
subcarrier signal f.sub.IF are arranged such that f.sub.LO
-f.sub.IF.gtoreq.1 [GHz] and 2.times.f.sub.IF <f.sub.LO
<(2/3).times.fr (resonant frequency of a laser) are satisfied.
With this arrangement, the RIN characteristics of the subcarrier signal
f.sub.IF can be prevented from degrading due to multiplex of the pilot
carrier signal f.sub.LO. Since satisfactory CNR characteristics can be
provided on the transmitting/receiving device side, the communication
quality of the transmitted optical signal is improved. When the above
conditions are satisfied, degradation in RIN characteristics can be
suppressed even when a large optical modulation index is set for the pilot
carrier signal. Since the optical modulation index of the pilot carrier
signal can be made large, the pilot carrier signal f.sub.LO with excellent
CNR characteristics can be provided on the transmitting/receiving device
side. Since the pilot carrier signal f.sub.LO is used by a multiplier as a
local oscillation signal for frequency conversion, an additive noise
amount in the output from the multiplier decreases, so a radio frequency
signal with few noise components can be obtained.
When the pilot carrier signal f.sub.LO is excellent in CNR characteristics,
the Q value of the filter for extracting the pilot carrier signal f.sub.LO
can be made small, so the frequency band of the pilot carrier signal
f.sub.LO to be transmitted becomes wide. That is, since the frequency
range of the radio frequency signal to be processed on the
transmitting/receiving device side is widened, a transmitting/receiving
station with a large application range can be provided.
On the transmitting/receiving device side, the pilot carrier signal
f.sub.LO transmitted from the transmitting/receiving station side can be
extracted with a high CNR. Therefore, the frequency of the radio frequency
signal can be up- or down-converted while suppressing the additive noise
amount. Since the degradation in CNR characteristics of the subcarrier
signal and pilot carrier signal is small, the optical transmission
distance between the transmitting/receiving station and
transmitting/receiving device can be increased. More specifically, the
setting range of a transmitting/receiving device connected to one
transmitting/receiving station can increase, the number of
transmitting/receiving devices which can be connected can be increased,
and the radio communication service area can be efficiency expanded.
Except the RIN value, a modulation distortion also degrades the CNR
characteristics. The frequency band with good laser modulation distortion
characteristics is lower than 1 [GHz]. Hence, when f.sub.IF <1 [GHz]
and 2 [GHz]<f.sub.LO are satisfied, a transmission system in which
degradation in CNR due to not only the RIN value but also modulation
distortion is suppressed can be provided.
According to the present invention, when a plurality of
transmitting/receiving devices are connected to a transmitting/receiving
station through a PON, the frequency stability between the
transmitting/receiving devices can be maintained using a simpler optical
transmission system. More specifically, the frequency of a radio wave is
set such that when data signals subcarrier-multiplexed are to be
distributed from a transmitting/receiving station to a plurality of
transmitting/receiving devices, the data signals to be used by the
transmitting/receiving devices are subcarrier-multiplexed at a
sufficiently large frequency interval so that the data signals can be
separated by a simple filter after reception of an optical signal, and
only two pilot carrier signals suffice to synchronize the frequencies of
radio waves radiated from the transmitting/receiving devices
(independently of the number of transmitting/receiving devices). As a
consequence, an optical communication system in which while establishing
frequency synchronization between the transmitting/receiving devices,
satisfactory transmission can be performed without sacrificing the optical
modulation index of the data signal in optical subcarrier transmission due
to transmission of the pilot carrier signal, and the process of extracting
necessary signals after reception of an optical signal is easy and
inexpensive can be provided.
Additional objects and advantages of the present invention will be set
forth in the description which follows, and in part will be obvious from
the description, or may be learned by practice of the present invention.
The objects and advantages of the present invention may be realized and
obtained by means of the instrumentalities and combinations particularly
pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
present invention and, together with the general description given above
and the detailed description of the preferred embodiments given below,
serve to explain the principles of the present invention in which:
FIG. 1 is a graph showing the frequency arrangement of a conventional
optical communication system;
FIG. 2 is a graph showing the optical modulation index vs. RIN
characteristics of a pilot carrier signal f.sub.LO in the conventional
frequency arrangement;
FIG. 3 shows a system arrangement of a conventional passive optical
network;
FIG. 4 shows another system arrangement of a conventional passive optical
network;
FIG. 5 shows the subcarrier multiplex of the conventional passive optical
network;
FIG. 6 is the block diagram showing the arrangement of a transmitting
station in an optical communication system according to a first embodiment
of the present invention;
FIG. 7 is a graph showing the frequency arrangement of an intermediate
frequency subcarrier signal f.sub.IF and a pilot carrier signal f.sub.LO
in the first embodiment;
FIG. 8 is a graph showing the RIN vs. frequency characteristics of the
pilot carrier signal f.sub.LO for the intermediate frequency subcarrier
signal f.sub.IF =1 [GHz];
FIG. 9 is a graph showing the RIN vs. frequency characteristics of the
pilot carrier signal f.sub.LO for the intermediate frequency subcarrier
signal f.sub.IF =120 [GHz];
FIG. 10 is a block diagram showing the arrangement of an optical
communication system according to a second embodiment of the present
invention;
FIG. 11 is a block diagram showing the arrangement of a transmitting device
in an optical communication system according to a third embodiment of the
present invention;
FIG. 12 is a block diagram showing the arrangement of a transmitting device
in an optical communication system according to a fourth embodiment of the
present invention;
FIG. 13 is a block diagram showing the arrangement of a transmitting device
in an optical communication system according to a fifth embodiment of the
present invention;
FIG. 14 is a block diagram showing the arrangement of a modification of the
fifth embodiment of the present invention;
FIG. 15 is a block diagram showing the system arrangement of a down link
system in an optical communication system according to a sixth embodiment
of the present invention;
FIG. 16 is a graph for explaining subcarrier multiplex in the sixth
embodiment;
FIG. 17 is a graph showing a specific frequency arrangement of pilot
carrier signals f.sub.LO of two types in an optical communication system
according to a seventh embodiment of the present invention;
FIG. 18 is a block diagram showing an arrangement of a frequency converter
in the seventh embodiment;
FIG. 19 is a block diagram showing another arrangement of the frequency
converter in the seventh embodiment;
FIG. 20 is a block diagram showing still another arrangement of the
frequency converter in the seventh embodiment;
FIG. 21 is a block diagram showing an arrangement of a frequency multiplier
in the frequency converter in the seventh embodiment;
FIG. 22 is a block diagram showing still another arrangement of the
frequency converter in the seventh embodiment;
FIG. 23 is a graph showing a specific frequency arrangement of pilot
carrier signals f.sub.LO of two types in an optical communication system
according to an eighth embodiment of the present invention;
FIG. 24 is a block diagram showing the arrangement of a
transmitting/receiving device having a plurality of antennas as a
modification of the eighth embodiment;
FIG. 25 is a block diagram showing the arrangement of a reception system
(up link system) in an optical communication system according to a ninth
embodiment of the present invention;
FIG. 26 is a graph showing an intermediate frequency subcarrier for each
transmitting/receiving device in the ninth embodiment;
FIG. 27 is a block diagram showing the arrangement of a
transmitting/receiving device having a plurality of antennas as a
modification of the ninth embodiment;
FIG. 28 is a block diagram showing an arrangement of a
transmitting/receiving device in an optical communication system according
to a tenth embodiment of the present invention;
FIG. 29 is a block diagram showing another arrangement of the
transmitting/receiving device in the optical communication system
according to the tenth embodiment of the present invention;
FIG. 30 is a block diagram showing still another arrangement of the
transmitting/receiving device in the tenth embodiment; and
FIG. 31 is a block diagram showing still another arrangement of the
transmitting/receiving device (subcarrier multiplex type
transmitting/receiving device) in the tenth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of an optical communication system according to the
present invention will now be described with reference to the accompanying
drawings.
First Embodiment
FIG. 6 shows the arrangement of a transmitting station as part of the first
embodiment of the present invention. A transmitting station 10 comprises a
modulator 12, an oscillator 14, an adder 16, and an electro-optical
converter (E/O) converter 18.
The modulator 12 modulates an intermediate frequency signal, which is
output from an oscillator (not shown), with a data signal to be
transmitted and supplies an intermediate frequency subcarrier signal
f.sub.IF as the modulation result to the first input terminal of the adder
16. The adder 16 adds the intermediate frequency subcarrier signal
f.sub.IF to a pilot carrier signal f.sub.LO output from the oscillator 14.
The E/O converter 18 converts the sum signal of the intermediate frequency
subcarrier signal f.sub.IF and pilot carrier signal f.sub.LO into an
optical signal. The E/O converter 18 comprises a driver amplifier 20, a
current source 22, a semiconductor laser element 24, an inductor 26, and a
resistor 28. The inductor 26 applies a bias corresponding to the output
from the current source 22 to an output signal from the driver amplifier
20. The resistor 28 is an input resistor for supplying the biased output
from the driver amplifier 20 to the semiconductor laser element 24 as a
direct modulation signal. The semiconductor laser element 24 emits a laser
beam modulated in correspondence with the output from the adder 16 and
sends the laser beam to an optical fiber 30 as a transmission line. The
semiconductor laser element 24 used is a DFB laser diode for analog
transmission.
The laser beam output from the semiconductor laser element 24 is
transmitted to a transmitting device (not shown) through the optical fiber
30. The resonant frequency of the semiconductor laser element 24 is
represented by fr.
In this arrangement, a condition required for the frequency arrangement
relationship between the intermediate frequency subcarrier signal f.sub.IF
and pilot carrier signal f.sub.LO is f.sub.LO -f.sub.IF.gtoreq.1 [GHz].
FIG. 7 shows the frequency arrangement of the intermediate frequency
subcarrier signal f.sub.IF and the pilot carrier signal f.sub.LO.
Normally, the lower the frequency band becomes, the more excellent the
modulation distortion characteristics and RIN characteristics of the
semiconductor laser element 24 become. Therefore, the intermediate
frequency subcarrier signal f.sub.IF modulated with the data to be
transmitted is arranged on the lower side of the pilot carrier signal
f.sub.LO. The pilot carrier signal f.sub.LO is a sinusoidal wave and can
stand the distortion.
FIG. 8 shows the RIN characteristics of the intermediate frequency
subcarrier signal f.sub.IF with respect to the frequency of the pilot
carrier signal f.sub.LO in the semiconductor laser element 24 in the
electro-optical converter 18. The intermediate frequency subcarrier signal
f.sub.IF has a frequency of 1 [GHz]. The optical modulation index of the
pilot carrier signal f.sub.LO is 40 [%]. The frequency of the intermediate
frequency subcarrier signal f.sub.IF is changed to 1.2 [GHz], 2 [GHz], 3
[GHz], 3.5 [GHz], and 4 [GHz]. The RIN obtained when the pilot carrier
signal f.sub.LO is not superposed is -152 [dB/Hz]. The RIN value is
influenced by the spectral component of the pilot carrier signal f.sub.LO.
As the frequency becomes close to the pilot carrier signal f.sub.LO, the
degradation becomes large.
When the frequency is separated from the pilot carrier signal f.sub.LO, the
RIN value is improved. As shown in FIG. 8, when f.sub.LO
-f.sub.IF.gtoreq.1 [GHz], the RIN value is asymptotic to the value "-152"
obtained when the pilot carrier signal f.sub.LO is not superposed, and
stabilizes.
FIG. 9 shows the RIN value to the pilot carrier signal f.sub.LO when the
intermediate frequency subcarrier signal f.sub.IF has a frequency of 120
[MHz]. The optical modulation index of the pilot carrier signal f.sub.LO
is 40 [%], as in FIG. 8. The RIN value obtained when the pilot carrier
signal f.sub.LO is not superposed is -164.0 [dB/Hz].
As is apparent from FIG. 9, when the pilot carrier signal f.sub.LO is close
to the intermediate frequency subcarrier signal f.sub.IF, and the
difference between the two signals is 1 [GHz] or less (i.e., when f.sub.LO
-f.sub.IF <1 [GHz]), the RIN value is -160 [dB/Hz], and the degradation
is great. For a pilot carrier signal f.sub.LO =2 [GHz] satisfying f.sub.LO
-f.sub.IF.gtoreq.1 [GHz], the RIN value becomes -162 [dB/Hz], and the
degradation is apparently suppressed.
As is apparent from the above description, when f.sub.LO -f.sub.IF.gtoreq.1
[GHz], the RIN characteristics can be improved.
Normally, the semiconductor laser element 24 has non-linear E/O conversion
characteristics. When a laser element is modulated directly by a sum
signal of f.sub.LO and f.sub.IF, intermodulation components are appeared
at frequency bands f.sub.LO -f.sub.IF and f.sub.LO +f.sub.IF. If the
intermodulation components overlap f.sub.IF and a resonant frequency
f.sub.r, the noise characteristic for f.sub.IF is distorted. It is because
the characteristics of laser elements becomes unstable by modulating the
resonant frequency f.sub.r and RIN increased through the signal frequency
band.
Therefore, the pilot carrier signal f.sub.LO must be arranged such that the
frequency f.sub.LO +f.sub.IF of the higher-band side distortion becomes
lower than the resonant frequency fr and the frequency f.sub.LO -f.sub.IF
of the lower-band side distortion becomes higher than the signal frequency
f.sub.IF. Since 2.times.f.sub.IF <f.sub.LO, and f.sub.LO +f.sub.IF
<fr, f.sub.LO <(2/3).times.fr. FIG. 7 shows the frequency
arrangement of the intermediate frequency subcarrier signal f.sub.IF and
pilot carrier signal f.sub.LO, which satisfies these conditions.
As the semiconductor laser element 24, i.e., the semiconductor laser diode,
a distributed feedback semiconductor laser (DFB-LD) or Fabri-Perot
semiconductor laser element (FP-LD) is used. Especially, a distributed
feedback semiconductor laser element has a small modulation distortion
that suppresses the dynamic range of a multi-channel signal, and is
suitable for analog transmission. However, even in the distributed
feedback semiconductor laser element, the frequency with a small
modulation distortion and noise amount is normally 1 [GHz] or less.
When the intermediate frequency subcarrier signal f.sub.IF as an
intermediate frequency subcarrier signal is arranged within the range of
f.sub.IF >1 [GHz], the dynamic range is suppressed because of
degradation in modulation distortion characteristics and an increase in
noise. Therefore, the intermediate frequency subcarrier signal f.sub.IF is
preferably arranged within the range of f.sub.IF <1 [GHz].
As is apparent from the RIN value when f.sub.IF =1 [GHz], which is shown in
FIG. 8 as the graph showing the pilot carrier signal f.sub.LO frequency
vs. RIN characteristics, and the RIN value when f.sub.IF =120 [MHz], which
is shown in FIG. 9 as the graph showing the pilot carrier signal f.sub.LO
frequency vs. RIN characteristics in the relatively high frequency band,
the RIN value on the frequency band of the intermediate frequency
subcarrier signal f.sub.IF, 120 MHz, is smaller by about 10 dB/Hz than the
RIN value on the frequency band of f.sub.IF, 1 GHz.
As described above, when the frequency arrangement of the intermediate
frequency subcarrier signal f.sub.IF and pilot carrier signal f.sub.LO
satisfies f.sub.IF <1 [GHz] and f.sub.LO >2 [GHz], as shown in FIG.
7, satisfactory transmission characteristics can be maintained without any
influence of the RIN characteristics and modulation distortion.
As described above, as a characteristic feature of the first embodiment,
the intermediate frequency subcarrier signal f.sub.IF modulated with data
to be transmitted is added to the pilot carrier signal f.sub.LO as a
sinusoidal wave. The sum signal is electro-optically converted by directly
modulating the semiconductor laser element 24 having the resonant
frequency fr and transmitted to the down link optical fiber. The frequency
f.sub.IF of the intermediate frequency subcarrier signal and the frequency
f.sub.LO Of the pilot carrier signal satisfy f.sub.LO -f.sub.IF.gtoreq.1
[GHz] and 2.times.f.sub.IF <f.sub.LO <(2/3).times.fr.
Normally, the lower the frequency band becomes, the more excellent the
modulation distortion characteristics and RIN characteristics of the
semiconductor laser element become. Therefore, when the intermediate
frequency subcarrier signal f.sub.IF is arranged on the lower side of the
pilot carrier signal f.sub.LO, the pilot carrier signal f.sub.LO as a
sinusoidal wave can stand the distortion.
In the RIN characteristics in the f.sub.IF band with respect to the pilot
carrier signal f.sub.LO in the semiconductor laser element, which are
shown in FIG. 8, the intermediate frequency subcarrier signal f.sub.IF has
a frequency of 1 [GHz]. The optical modulation index of the pilot carrier
signal f.sub.LO is 40 [%]. The frequency of the intermediate frequency
subcarrier signal f.sub.IF is changed to 1.2 [GHz], 2 [GHz], 3 [GHz], 3.5
[GHz], and 4 [GHz]. The RIN obtained when the pilot carrier signal
f.sub.LO is not superposed is -152 [dB/Hz]. The RIN value is influenced by
the spectral component of the pilot carrier signal f.sub.LO. As the
frequency becomes close to the pilot carrier signal f.sub.LO, the
degradation becomes large.
When the frequency is separated from the pilot carrier signal f.sub.LO, the
RIN value decreases. As shown in FIG. 8, when f.sub.LO -f.sub.IF.gtoreq.1
[GHz], the RIN value is asymptotic to the value "-152" obtained when the
pilot carrier signal f.sub.LO is not superposed, and stabilizes.
As shown in FIG. 9, in the RIN characteristics with respect to the pilot
carrier signal f.sub.LO when the intermediate frequency subcarrier signal
f.sub.IF has a frequency of 120 [MHz], the optical modulation index of the
pilot carrier signal f.sub.LO is 40 [%], as in FIG. 8. The RIN value
obtained when the pilot carrier signal f.sub.LO is not superposed is
-164.0 [dB/Hz].
As is apparent from FIG. 9, when the pilot carrier signal f.sub.LO is close
to the intermediate frequency subcarrier signal f.sub.IF, and the
difference between the two signals is 1 [GHz] or less (i.e., when f.sub.LO
-f.sub.IF <1 [GHz]), the RIN value is equal to or larger than -160
[dB/Hz], and the degradation is large. However, when the pilot carrier
signal f.sub.LO maintains the relation to the f.sub.IF such that f.sub.LO
-f.sub.IF.gtoreq.1 [GHz], the degradation is suppressed.
Hence, when the arrangement satisfies f.sub.LO -f.sub.IF.gtoreq.1 [GHz],
the RIN characteristics can be improved.
Normally, the semiconductor laser element has non-linear E/O conversion
characteristics. When the laser element is directly modulated, a frequency
corresponding to f.sub.LO.+-.f.sub.IF has an intermodulation distortion
between the signals f.sub.LO and f.sub.IF, resulting in an increase in
noise. Therefore, it is important to arrange the pilot carrier signal
f.sub.LO with respect to the intermediate frequency subcarrier signal
f.sub.IF in consideration of f.sub.LO.+-.f.sub.IF. When the frequency
f.sub.LO -f.sub.IF of the lower-band side distortion overlaps the
frequency band of the intermediate frequency subcarrier signal f.sub.IF,
the RIN characteristics in the peripheral band degrade, as described
above.
To prevent this, f.sub.LO >2.times.f.sub.IF is set to satisfy f.sub.LO
-f.sub.IF >f.sub.IF, thereby avoiding the influence of degradation in
RIN characteristics. In addition, the semiconductor laser element has the
resonant frequency fr which is a specific frequency for each laser
element.
Modulation efficiency at the resonant frequency fr band becomes much large
comparing with the lower frequency band than fr. The resonant frequency is
explained under. If a laser diode is suddenly turned-on from zero bias, a
turn-on delay and an exponential rise in the optical output will be
observed. The optical output initially overshoots and goes through a few
cycles of damped oscillation before reaching equilibrium. The oscillation
frequency of this behavior is called "resonant frequency fr". This
behavior is caused by the inverse relationship between carrier density and
photon density in the semiconductor. If the resonant frequency fr is
modulated, the characteristics of the laser element become unstable, and
the RIN increases throughout the frequency band. Therefore, the pilot
carrier signal f.sub.LO must be arranged such that the frequency f.sub.LO
+f.sub.IF of the higher-band side distortion becomes lower than the
resonant frequency fr. Since 2.times.f.sub.IF <f.sub.LO, and f.sub.LO
+f.sub.IF <fr, f.sub.LO <(2/3).times.fr.
As another characteristic feature of the first embodiment, the frequency
band f.sub.IF of the intermediate frequency subcarrier signal satisfies is
lower than 1 [GHz], and the frequency band f.sub.LO of the pilot carrier
signal f.sub.LO is higher than 2 [GHz].
As the semiconductor laser element, i.e., the semiconductor laser diode, a
distributed feedback semiconductor laser (DFIB-LD) or Fabri-Perot
semiconductor laser element (FP-LD) is used. Especially, a DFB-LD has a
small modulation distortion that suppresses the dynamic range of a
multi-channel signal, and is suitable for analog transmission. However,
even in the DFB-LD, the frequency band with a small modulation distortion
and noise amount is normally 1 [GHz] or less.
When the frequency f.sub.IF of the intermediate frequency subcarrier signal
f.sub.IF as an intermediate frequency subcarrier signal is arranged within
the range of f.sub.IF >1 [GHz], the dynamic range is suppressed because
of degradation in modulation distortion characteristics and an increase in
noise. Therefore, the intermediate frequency subcarrier signal f.sub.IF is
preferably arranged within the range of f.sub.IF <1 [GHz].
As is apparent from the RIN value when f.sub.IF =1 [GHz], which is shown in
FIG. 8 as the graph showing the pilot carrier signal f.sub.LO frequency
vs. RIN characteristics in the relatively low frequency band, and the RIN
value when f.sub.IF =120 [MHZ], which is shown in FIG. 9 as the graph
showing the pilot carrier signal f.sub.LO frequency vs. RIN
characteristics in the relatively high frequency band, the RIN value on
the frequency band of the intermediate frequency subcarrier signal
f.sub.IF, 120 [MHz], is smaller by about 10 dB/Hz than the RIN value on
the frequency band of f.sub.IF, 1 [GHz].
As described above, when the frequency arrangement of the intermediate
frequency subcarrier signal f.sub.IF and pilot carrier signal f.sub.LO
satisfies f.sub.IF <1 [GHz] and f.sub.LO.gtoreq.2 [GHz], satisfactory
transmission characteristics can be maintained without any influence of
the RIN characteristics and modulation distortion.
Other embodiments of the optical transmission apparatus according to the
present invention will be described. The same portions as those of the
first embodiment will be indicated in the same reference numerals and
their detailed description will be omitted.
Second Embodiment
FIG. 10 shows an optical communication system according to the second
embodiment of the present invention. The second embodiment is associated
with an entire optical communication system including the transmitting
station of the first embodiment of the present invention, and a radio
communication base station device including a transmitting device.
As shown in FIG. 10, a transmitting station 10 is connected to transmitting
devices 32-1, 32-2, . . . through an optical fiber 30.
Each transmitting device 32 is connected to the optical fiber 30 through an
optical divider 34. The transmitting devices 32 are set at separate
locations. A range where radio waves reach is a radio zone (cell or
service area), and each transmitting device 32 can transmit/receive radio
waves to/from communication terminals in the cell.
The transmitting station 10 is the same as in the first embodiment shown in
FIG. 6.
The optical fiber 30 is an optical transmission line connecting an E/O
converter 18 in the transmitting station 10 to an opto-electrical
converter (O/E converter) 34 in each transmitting device 32.
Each transmitting device 32 comprises the O/E converter 34, a divider 36,
bandpass filters 38 and 40, a multiplier 42, a bandpass filter 44, a power
a emplifier 46, and a transmission antenna 48.
The O/E converter 34 receives an optical signal transmitted through the
optical fiber 30 and converts the optical signal into an electrical
signal. The divider 36 receives the electrical signal output from the
optical divider 34 and supplies it to the bandpass filters 38 and 40.
The bandpass filter 38 extracts an intermediate frequency subcarrier signal
f.sub.IF and contains the frequency f.sub.IF in the passband. The bandpass
filter 40 extracts a pilot carrier signal f.sub.LO and contains the
frequency f.sub.LO in the passband.
The multiplier 42 multiplies output signals from the two bandpass filters
38 and 40 and outputs the multiplied signal. The bandpass filter 44
extracts a predetermined radio frequency signal from the output from the
multiplier 42. The power amplifier 46 power-amplifies the radio frequency
signal output from the bandpass filter 44. The antenna 48 radiates the
amplified signal into air as a radio wave.
In the system with the above arrangement, a radio data signal obtained by
adding the pilot carrier signal f.sub.LO from a local oscillator 14 to the
intermediate frequency subcarrier signal f.sub.IF from a radio signal
modulator 12 by an adder 16 in the transmitting station 10 is input to an
E/O converter 18.
The E/O converter 18 directly modulates a laser beam with the radio data
signal to obtain an optical signal. This optical signal is transmitted to
the transmitting devices 32-1, 32-2, . . . through the optical fiber 30.
On the side of each of the transmitting devices 32-1, 32-2, . . . , the
optical signal transmitted through the optical fiber 30 is received by the
O/E converter 34, converted into an electrical signal and separated into
two paths by the divider 36. One is supplied to the bandpass filter 38
having the passband for the intermediate frequency subcarrier signal
f.sub.IF, and the other is supplied to the bandpass filter 40 having the
passband for the pilot carrier signal f.sub.LO, thereby reproducing the
original intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO.
The reproduced intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO are input to the multiplier 42 and multiplied.
The output from the multiplier 42 is passed through the bandpass filter 44
to extract a predetermined radio frequency signal. The extracted radio
frequency signal is amplified through the power amplifier 46, radiated
from the transmission antenna 48 into air as a radio wave, and transmitted
to a terminal side in the cell.
According to the second embodiment, the frequency arrangement of the
intermediate frequency subcarrier signal f.sub.IF and pilot carrier signal
f.sub.LO is set to satisfy f.sub.LO -f.sub.IF.gtoreq.1 [GHz] and
2.times.f.sub.IF <f.sub.LO <(2/3).times.fr, or f.sub.IF <1 [GHz]
and f.sub.LO >2 [GHz], as in the first embodiment.
In the above-described manner, the first embodiment can be applied to the
base station device for radio communication. The intermediate frequency
subcarrier signal f.sub.IF is a single-channel or a frequency-division
multiplexed signal. In case the intermediate subcarrier signal f.sub.IF is
a frequency-division multiplexed signal, each channel frequency of
f.sub.IF may be changed for each unit of transmitting device, or the same
frequency of f.sub.IF may be used.
The second embodiment of the present invention, in which the first
embodiment is applied to the base station device for radio communication,
has been described above. Next, the third embodiment in which transmitting
devices with the same specifications are used between adjacent cells
against the different frequency band of an intermediate frequency signal,
a pilot carrier signal and a radio frequency signal, thereby reducing cost
of the system without exchanging the hardware for each transmitting
device.
Third Embodiment
FIG. 11 shows the third embodiment of the present invention. The
transmitting station 10 has the same arrangement as in the first and
second embodiments. In the transmitting device 32 as well, the same
reference numerals as in the transmitting device 32 of the second
embodiment denote the same parts in the third embodiment.
The transmitting device 32 of the third embodiment comprises the O/E
converter 34, the divider 36, the multiplier 42, the bandpass filter 44,
the power amplifier 46, the antenna 48, a low-pass filter 52, and a
high-pass filter 54.
In this system as well, in the transmitting station 10, a radio data signal
obtained by adding the pilot carrier signal f.sub.LO from the local
oscillator 14 to the intermediate frequency subcarrier signal f.sub.IF
from the radio signal modulator 12 by the adder 16 is input to the E/O
converter 18, a semiconductor laser element in the E/O converter 18 is
directly modulated by the radio data signal to obtain an optical signal,
and this optical signal is transmitted to the transmitting device 32
through the optical fiber 30, as in the first and second embodiments.
Of the constituent elements of the transmitting device 32, the O/E
converter 34 receives the optical signal transmitted from the transmitting
station 10 through the optical fiber 30 and converts the optical signal
into an electrical signal. The divider 36 supplies the electrical signal
output from the O/E converter 34 to the low-pass filter 52 and the
high-pass filter 54.
The low-pass filter 52 has the passband of the intermediate frequency
subcarrier signal f.sub.IF, and the high-pass filter 54 has the passband
of the pilot carrier signal f.sub.LO.
The multiplier 42 multiples output signals from the two filters 52 and 54
and outputs the multiplied signal. The bandpass filter 44 extracts a
predetermined radio frequency signal from the output from the multiplier
42. The power amplifier 46 power-amplifies the radio frequency signal
output from the bandpass filter 44 and outputs the amplified signal. The
antenna 48 radiates the amplified signal into air as a radio wave.
In the system with the above arrangement, on the transmitting device side,
the optical signal transmitted through the optical fiber 30 is received by
the O/E converter 34, converted into an electrical signal, and supplied
through the divider 36 to the low-pass filter 52 having the passband for
the intermediate frequency subcarrier signal f.sub.IF and the high-pass
filter 54 having the passband for the pilot carrier signal f.sub.LO,
thereby reproducing the original intermediate frequency subcarrier signal
f.sub.IF and pilot carrier signal f.sub.LO.
The reproduced intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO are input to the multiplier 42 and multiplied.
The output from the multiplier 42 is passed through the bandpass filter 44
to extract a predetermined radio frequency signal. The extracted radio
frequency signal is amplified through the power amplifier 46, radiated
into air as a radio wave through the antenna 48, and transmitted to a
terminal side in the cell.
As described above, in the system according to the third embodiment, on the
transmitting device 32 side, two outputs from the divider 36 are input to
the low-pass filter 52 and high-pass filter 54, respectively, to extract
the intermediate frequency subcarrier signal f.sub.IF and pilot carrier
signal f.sub.LO. That is, not the bandpass filters 38 and 40 but the
low-pass filter 52 and high-pass filter 54 are used to extract the
intermediate frequency subcarrier signal f.sub.IF and pilot carrier signal
f.sub.LO.
According to this arrangement, if the passband of the bandpass filter 44
connected to the output side to extract a radio frequency signal has a
margin, the flexibility for frequency selection increases so that the
carrier of the radio frequency signal can be changed without exchanging
the hardware of the transmitting device since the low-pass filter 52 and
high-pass filter 54 have wider frequency passbands than the passband of
the bandpass filter.
To extract the original pilot carrier signal f.sub.LO and intermediate
frequency subcarrier signal f.sub.IF from the sum signal of the pilot
carrier signal f.sub.LO and intermediate frequency subcarrier signal
f.sub.IF using two bandpass filters, bandpass filters having large Q
values depending on the CNR value of the signal to be processed are
normally used. More specifically, to prevent any decrease in CNR value of
a received signal obtained by receiving an optical signal from the
transmitting station 10 through the optical fiber 30 and photoelectrically
converting the signal by the O/E converter 34, filters having large Q
values must be used as the bandpass filters 38 and 40, and the range of
selection of the frequency of a radio frequency signal becomes narrow.
However, according to the system of the third embodiment, even when the
pilot carrier signal f.sub.LO is superposed on the transmitting station 10
side, an increase in RIN value of the intermediate frequency subcarrier
signal f.sub.IF band can be suppressed. Hence, the optical modulation
index of the pilot carrier signal f.sub.LO can be made large without
increasing the RIN value of the intermediate frequency subcarrier signal
f.sub.IF band, and the intermediate frequency subcarrier signal f.sub.IF
and pilot carrier signal f.sub.LO can be separated using the low-pass
filter 52 and high-pass filter 54. Therefore, flexibility of the radio
frequency signal can be increased, so a radio communication station with
a-wide application range can be provided.
A fourth embodiment in which the frequency of a radio frequency signal can
be highly increased even when the frequencies of intermediate frequency
subcarrier signal f.sub.IF and pilot carrier signal f.sub.LO are not high.
Fourth Embodiment
FIG. 12 shows the fourth embodiment of the present invention. The
arrangement of a transmitting station 10 is the same as in the first and
second embodiments. In a transmitting device 32 as well, the same
reference numerals as in the first and second embodiments denote the same
parts in the fourth embodiment.
The transmitting device 32 of the fourth embodiment is different from the
transmitting device 32 of the third embodiment in that a multiplier 56 and
a bandpass filter 58 are connected between the high-pass filter 54 and the
multiplier 42. The multiplier 56 is multiples a filtered output from the
high-pass filter 54 by n and outputs the signal. The bandpass filter 58
extracts a predetermined frequency band component from the output
multiplied by n. Note that n is a positive integer.
In the system having the above arrangement, on the transmitting device 32
side, an optical signal transmitted through the optical fiber 30 is
received by the O/E converter 34, converted into an electrical signal. The
electrical signal is supplied through the divider 36 to the low-pass
filter 52 having the passband for an intermediate frequency subcarrier
signal f.sub.IF and the high-pass filter 54 having the passband for a
pilot carrier signal f.sub.LO, thereby reproducing the original
intermediate frequency subcarrier signal f.sub.IF and pilot carrier signal
f.sub.LO.
Of the reproduced intermediate frequency subcarrier signal f.sub.IF and
pilot carrier signal f.sub.LO, the pilot carrier signal f.sub.LO is
multiplied by n by the multiplier 56 and then passed through the bandpass
filter 58 to obtain a pilot carrier signal n.times.f.sub.LO multiplied by
a desired value. This signal is input to the multiplier 42 and used for
frequency conversion.
The multiplier 42 multiplies the intermediate frequency subcarrier signal
f.sub.IF from the low-pass filter 52 by the multiplied pilot carrier
signal n.times.f.sub.LO. The obtained signal output is passed through a
bandpass filter 44 to extract a predetermined radio frequency signal. The
radio frequency signal output from the bandpass filter 44 is
power-amplified by the power amplifier 46 and radiated from the antenna 48
into air as a radio wave.
In the fourth embodiment, the multiplier 56 and bandpass filter 58 are
added to the arrangement of the third embodiment. The filtered output from
the high-pass filter 54 is multiplied by n, and a predetermined frequency
band component is extracted, by the bandpass filter 58, from the output
multiplied by n, thereby obtaining the pilot carrier signal
n.times.f.sub.LO multiplied by a desired value, which is to be used for
frequency conversion. This point is different from the third embodiment.
As a laser light source used in the electro-optical converter 18 of the
transmitting station 10, a distributed feedback semiconductor laser
element (DFB-LD) or a Fabri-Perot semiconductor laser element (FP-LD) is
used.
Normally, a modulation band fc of a DFB-LD is 3 [GHz], and that of an FP-LD
is 1 to 2 [GHz]. If the frequency band becomes higher, the modulation
efficiency degrades. Therefore, except for a special processed laser
element which can be modulated by a high frequency signal, the frequency
band of the pilot carrier signal f.sub.LO that can be superposed is
limited to about 3 to 5 [GHz].
Since the frequency of the intermediate frequency subcarrier signal
f.sub.IF used in this system is lower than 1 [GHz], a frequency f.sub.MW
of a radio frequency signal corresponding to f.sub.IF +f.sub.LO (output
from the bandpass filter 44) is limited to 4 to 6 [GHz].
However, in the arrangement having the multiplier 56, as shown in FIG. 12,
the pilot carrier signal f.sub.LO to be used for frequency conversion is
multiplied by a desired value. Since the frequency fMw of the radio
frequency signal can be set as f.sub.MW =f.sub.IF +n.times.f.sub.LO, this
system can generate a radio frequency signal in a higher frequency band
without being limited to the modulation band of the laser element in the
electro-optical converter 18.
However, noise is inevitably added upon multiplying the pilot carrier
signal f.sub.LO by n, and this normally degrades the quality of the radio
frequency signal. According to the present invention, however, the optical
modulation index of the pilot carrier signal f.sub.LO can be made large
without increasing the RIN value of the intermediate frequency subcarrier
signal f.sub.IF band, so both the pilot carrier signal and subcarrier
signal can maintain satisfactory CNR characteristics.
Therefore, the system of the fourth embodiment can prevent any large
degradation in quality of the radio frequency signal even when the
multiplier 56 is used. In place of the low-pass filter 52 and high-pass
filter 54, bandpass filters 38 and 40 may be used, as in the first and
second embodiments.
According to the fourth embodiment, the transmitting device comprises an
opto-electrical converter for receiving an optical signal transmitted
through an optical fiber, converting the optical signal into an electrical
signal, and outputting the electrical signal, a filter for extracting the
intermediate frequency subcarrier signal f.sub.IF and pilot carrier signal
f.sub.LO from the converted and output electrical signal, a frequency
multiplier for multiplying the extracted pilot carrier signal f.sub.LO, a
frequency converter for frequency-converting the extracted intermediate
frequency subcarrier signal f.sub.IF using the multiplied pilot carrier
signal f.sub.LO to obtain a radio frequency signal, and an antenna for
transmitting the obtained radio frequency signal.
On the transmitting station side, the pilot carrier signal f.sub.LO as a
sinusoidal wave used to up-converting the intermediate frequency
subcarrier signal f.sub.IF into a radio frequency signal (frequency
F.sub.0) is added to the intermediate frequency subcarrier signal f.sub.IF
as a signal in the intermediate frequency band, which is modulated with
data to be transmitted. The sum signal is converted into an optical signal
and sent to the optical fiber. This conversion to an optical signal is
performed by controlling a current from a semiconductor laser element in
accordance with the sum signal.
As the semiconductor laser element, a distributed feedback semiconductor
laser element (DFB-LD) or a Fabri-Perot semiconductor laser element
(FP-LD) is used. Normally, the modulation band fc of a DFB-LD is 3 [GHz],
and that of a FP-LD is 1 to 2 [GHz]. If the frequency band becomes higher,
the modulation efficiency degrades. Therefore, except for a special
processed laser element which can be modulated by a high frequency signal,
the frequency band of the pilot carrier signal f.sub.LO that can be
superposed is limited to about 3 to 5 [GHz].
Since the frequency of the intermediate frequency subcarrier signal
f.sub.IF used in this embodiment is lower than 1 [GHz], the frequency
F.sub.0 of a radio frequency signal corresponding to f.sub.IF +f.sub.LO is
originally limited to 4 to 6 [GHz].
However, when a multiplier is used, the pilot carrier signal f.sub.LO to be
used for frequency conversion is multiplied. Since the frequency F.sub.0
of the radio frequency signal can be set as F.sub.0 =f.sub.IF
+n.times.f.sub.LO (n is a positive integer), a radio frequency signal in a
higher frequency band can be generated without being limited to the
modulation band of the laser element in the electro-optical converter 18.
The above embodiments have been described mainly in association with
transmission. An actual system need allow transmission and reception. The
fifth embodiment for such a system will be described next.
Fifth Embodiment
FIGS. 13 and 14 show the fifth embodiment of the present invention. The
main arrangement is the same as in the second embodiment, and the same
reference numerals as in the above embodiments denote the same parts in
the fifth embodiment. In the fifth embodiment, the present invention is
applied to a base station device for radio communication, as in the second
embodiment.
In the embodiment shown in FIG. 13, to allow bi-directional communication,
the terminal station is constructed as a transmitting/receiving station
10A including not only the transmission function but also the reception
function. The transmitting device connected to the transmitting/receiving
station 10A is also constructed as a transmitting/receiving device 32A
having not only the transmission function but also the reception function.
The transmitting/receiving station 10A and transmitting/receiving device
32A are connected through optical fibers 30a and 30b. Although FIG. 13
illustrates only one transmitting/receiving device 32A, a plurality of
transmitting/receiving devices 32A may be arranged in correspondence with
one transmitting/receiving station 10A. When a plurality of
transmitting/receiving devices 32A are present, the transmitting/receiving
device 32A are set at separate locations. A range where radio waves reach
is a radio zone (cell or service area), and each transmitting/receiving
device 32A can transmit/receive radio waves to/from communication
terminals in the cell.
Of the optical fibers 30a and 30b, the former is used for a down link (for
a transmission line), and the latter is used for an up link (for a
reception line).
The transmitting/receiving station 10A has the radio signal modulator 12,
the local oscillator 14, the adder 16, and the E/O converter 18 for the
down link (for transmission), and an O/E converter 62 and a demodulator 64
for the up link (for reception).
The transmitting/receiving device 32A has the O/E converter 34, the divider
36, the bandpass filters 38, 40, and 44, the multiplier 42, and the power
amplifier 46 for the down link (for transmission), a circulator (or
duplexer) 66, and a transmission/reception antenna 68. The
transmitting/receiving device 32A further includes a low-noise amplifier
70, a bandpass filter 72, a multiplier 74, a bandpass filter 76, and an
E/O converter 78 for the up link (for reception).
Of these elements, the E/O converter 18 incorporates a semiconductor laser
element as a light source and has a function of outputting an optical
signal modulated with a radio data signal output from the adder 16 by
controlling the current of the semiconductor laser element in accordance
with the radio data signal. The E/O converter 18 is connected to the
optical fiber 30a. The optical signal output from the E/O converter 18 is
output to the optical fiber 30a.
The O/E converter 34 as a constituent element of the transmitting/receiving
device 32A converts the optical signal transmitted through the optical
fiber 30a into an electrical signal. The divider 36 supplies the received
electrical signal output from the O/E converter 34 to the bandpass filters
38 and 40.
The bandpass filter 38 extracts an intermediate frequency subcarrier signal
f.sub.IF and contains the frequency f.sub.IF in the passband. The bandpass
filter 40 extracts a pilot carrier signal f.sub.LO and contains the
frequency f.sub.LO in the passband.
The multiplier 42 multiplies output signals from the two bandpass filters
38 and 40 and outputs the multiplied signal. The bandpass filter 44
extracts a predetermined radio frequency signal from the output from the
multiplier 42. The power amplifier 46 power-amplifies the radio frequency
signal output from the bandpass filter 44 and outputs the amplified
signal. The antenna 68 receives the amplified signal through the
circulator (or duplexer) 66 and radiates the signal into air as a radio
wave. The antenna 68 also receives a radio wave arriving from air and
supplies the signal to the low-noise amplifier 70 through the circulator
66.
The circulator 66 is a device for switching between the path for guiding
the radio frequency signal to be transmitted to the antenna 68 and the
path for guiding a received radio frequency signal received by the antenna
68 to the low-noise amplifier 70.
The low-noise amplifier 70 has performance for amplifying the received
radio frequency signal with low noise. The bandpass filter 72 passes an
output from the low-noise amplifier 70 through a predetermined passband to
extract a predetermined passband component. The multiplier 74 multiplies
the output from the bandpass filter 72 by the pilot carrier signal
f.sub.LO output from the bandpass filter 40.
The bandpass filter 76 passes the output from the multiplier 74 in a
predetermined passband to extract a predetermined passband component as a
radio data signal. The E/O converter 78 converts the radio data signal
obtained through the bandpass filter 76 into an optical signal and outputs
the optical signal. The E/O converter 78 incorporates a semiconductor
laser element as a light source and has a function of outputting an
optical signal modulated with the radio data signal by controlling the
current of the semiconductor laser element in accordance with the radio
data signal. The optical signal output from the E/O converter 78 is output
to the optical fiber 30b.
The O/E converter 62 of the transmitting/receiving station 10A is connected
to the optical fiber 30b to convert the optical signal transmitted from
the transmitting/receiving device 32A through the optical fiber 30b into
an electrical signal and outputs the signal. The demodulator 64 receives
the electrical signal converted by the O/E converter 62 and demodulates
the signal into the original radio data signal.
In the fifth embodiment having the above arrangement, in the
transmitting/receiving station 10A, a radio data signal obtained by adding
the pilot carrier signal f.sub.LO from the local oscillator 14 to the
intermediate frequency subcarrier signal f.sub.IF from the radio signal
modulator 12 by the adder 16 is input to the E/O converter 18.
In the E/O converter 18, the laser element is directly modulated with the
radio data signal to obtain an optical signal. This optical signal is
transmitted to the transmitting/receiving device 32A through the optical
fiber 30a.
On the transmitting/receiving device 32A side, the optical signal
transmitted through the optical fiber 30a is received by the O/E converter
34, converted into an electrical signal. The electrical signal is supplied
to the bandpass filter 38 having the passband for the intermediate
frequency subcarrier signal f.sub.IF and the bandpass filter 40 having the
passband for the pilot carrier signal f.sub.LO, thereby reproducing the
original intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO.
The reproduced intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO are input to the multiplier 42 and multiplied.
The output from the multiplier 42 is passed through the bandpass filter 44
to extract a predetermined radio frequency signal. The extracted radio
frequency signal is amplified through the power amplifier 46, radiated
from the antenna 68 into air through the circulator 66 as a radio wave,
and transmitted to a terminal side in the cell.
On the other hand, a radio wave transmitted from a terminal side in the
cell is received by the antenna 68, input to the low-noise amplifier 70
through the circulator 66, and amplified. A predetermined band component
is extracted by the bandpass filter 72. The extracted component signal is
multiplied by the pilot carrier signal f.sub.LO from the bandpass filter
40 by the multiplier 74 and down-converted. A predetermined band component
is extracted by the bandpass filter 76, converted into an optical signal
by the E/O converter 78, sent to the optical fiber 30b as an up link
optical signal, and sent to the transmitting/receiving station 10A.
As the characteristic feature of this embodiment, the pilot carrier signal
f.sub.LO is extracted in the transmitting/receiving device 32A using the
bandpass filter 40, the extracted pilot carrier signal f.sub.LO is input
to the multiplier 42 on the transmission system side and the multiplier 74
on the reception system side. In the transmission system, the extracted
pilot carrier signal f.sub.LO is multiplied by the intermediate frequency
subcarrier signal f.sub.IF by the multiplier 42 to up-convert the
frequency of the intermediate frequency subcarrier signal f.sub.IF. In the
reception system, the radio frequency signal is multiplied by the pilot
carrier signal f.sub.LO by the multiplier 74 to down-convert the frequency
of the radio frequency signal.
That is, the intermediate frequency subcarrier signal f.sub.IF and pilot
carrier signal f.sub.LO are extracted from two outputs from the divider 36
in the transmitting/receiving device 32A using the bandpass filters 38 and
40. The extracted pilot carrier signal f.sub.LO is separated from the
transmission system and also input to the multiplier 74 of the reception
system.
The multiplier 42 up-converts the frequency of the intermediate frequency
subcarrier signal f.sub.IF transmitted from the transmitting/receiving
station 10A using the pilot carrier signal f.sub.LO to obtain a radio
frequency signal, and this radio frequency signal is transmitted by radio
through the power amplifier 46 and antenna 68, as described above.
The multiplier 74 multiplies a radio frequency signal by the pilot carrier
signal f.sub.LO to down-convert the frequency of the radio frequency
signal.
More specifically, the radio frequency signal transmitted by radio is
received by the antenna 68 and input to the low-noise amplifier 70 through
the circulator or duplexer 66, and a desired band is extracted by the
bandpass filter 72. The frequency of the extracted band component of the
radio frequency signal is down-converted by the multiplier 74 using the
pilot carrier signal f.sub.LO. The image frequency and the like are
removed by the bandpass filter 76 to extract a desired band, thereby
obtaining an up link intermediate frequency signal. The up link
intermediate frequency signal is converted into an optical signal by the
E/O converter 78 and transmitted to the transmitting/receiving station 10A
through the optical fiber 30b.
In the transmitting/receiving station 10A, the optical signal transmitted
from the transmitting/receiving device 32A side is received by the O/E
converter 62 and input to the demodulator 64 to extract data.
As described above, in the fifth embodiment, in the transmitting/receiving
device 32A, the intermediate frequency subcarrier signal f.sub.IF and
pilot carrier signal f.sub.LO are extracted from the sum signal of the
intermediate frequency subcarrier signal f.sub.IF and pilot carrier signal
f.sub.LO for radio transmission, which are transmitted from the
transmitting/receiving station 10A, using the bandpass filters. The
extracted pilot carrier signal f.sub.LO is used for frequency up
conversion in the transmission system and for frequency down conversion in
the reception system.
Hence, the transmitting/receiving device 32A can down-converts the
frequency of an up link signal from the transmitting/receiving device 32A
to the transmitting/receiving station 10A without requiring a component
such as a local oscillator. Therefore, the constituent elements of the
reception system can be simplified. In addition, a received signal in a
radio frequency band, which is received by the antenna is down-converted,
input to the E/O converter 78, converted into an optical signal, and sent
to the transmitting/receiving station 10A. Therefore, the frequency band
required for the E/O converter 78 of the reception system can be made low.
Since the frequency band of the signal to be processed is low,
specifications of the laser element, driver amplifier, and the like
incorporated in the E/O converter 78 can be lenient, and inexpensive
elements can be used.
With the above arrangement, the transmitting/receiving device 32A can be
made compact and simple, so an inexpensive transmitting/receiving device
32A can be provided.
FIG. 14 shows another arrangement of the transmitting/receiving device 32A.
In the transmitting/receiving device 32A shown in FIG. 14, the multiplied
pilot carrier signal f.sub.LO is used for frequency conversion.
In the example, as in the fourth embodiment shown in FIG. 12, the
intermediate frequency subcarrier signal f.sub.IF is extracted from the
output from the divider 36 by the low-pass filter 52, the pilot carrier
signal f.sub.LO is extracted by the high-pass filter 54, the extracted
pilot carrier signal f.sub.LO is multiplied by n by the multiplier 56 and
passed through the multiplier 56, and this pilot carrier signal
n.times.f.sub.LO multiplied by a desired value is used for frequency up
conversion in the transmission system and frequency down conversion in the
reception system.
Except that the pilot carrier signal n.times.f.sub.LO multiplied by a
desired value is input to the multiplier 42 and used for up conversion or
input to the multiplier 74 and used for down conversion, the arrangement
is the same as in FIG. 13, and a detailed description thereof will be
omitted.
In this example, since the multiplier 56 and bandpass filter 58 are added,
the circuit scale becomes larger than that of the example shown in FIG.
13. Even with this arrangement, the frequency band required for the E/O
converter 78 in the reception system can be made low, as in the example
shown in FIG. 13. Specifications of the laser element, driver amplifier,
and the like incorporated in the E/O converter 78 can be lenient because
the frequency band of a signal to be processed becomes low. Hence,
inexpensive elements can be used. In addition, since the multiplier is
added, the pilot carrier signal f.sub.LO to be used for frequency
conversion is multiplied by a desired value. The frequency of a radio
frequency signal can be made higher by n, so a radio frequency signal in a
higher frequency band can be generated without being limited by the
modulation band in the E/O converter 18 in the transmitting/receiving
station 10A.
According to the fifth embodiment, even when the optical modulation index
of the pilot carrier signal f.sub.LO is increased, the CNR of the
intermediate frequency subcarrier signal f.sub.IF does not decrease.
Therefore, on the transmitting/receiving device 32A side, the pilot
carrier signal f.sub.LO with a satisfactory CNR can be obtained. In the
transmitting/receiving device 32A, the radio frequency signal received by
the antenna 68 is sometimes weak, and the signal for frequency conversion
by the multiplier 74 is required to have a high CNR. As the signal for
frequency conversion, the pilot carrier signal f.sub.LO from the
transmitting/receiving station 10A can be provided. Additionally, when the
pilot carrier signal f.sub.LO is multiplied as a signal for frequency
conversion, the noise characteristics are not largely degraded in
frequency conversion because the CNR of the received pilot carrier signal
f.sub.LO is large.
In place of the low-pass filter 52 and high-pass filter 54, the bandpass
filters 38 and 40 may be used, as in the first and second embodiments.
Sixth Embodiment
An embodiment in which when modulated and multiplexed subcarrier signals
are to be transmitted from a transmitting/receiving station to a plurality
of transmitting/receiving devices, the number of pilot carrier signals for
maintaining the frequency stability of radio waves to be radiated from the
transmitting/receiving devices is decreased, and a data signal to be
radiated can be separated from the received optical signal by a simple
filter in each transmitting/receiving device will be described. In this
case, only two pilot carrier signals are used regardless of the number of
the subcarrier signals, and subcarrier signals are multiplexed at a
sufficiently large frequency interval such that the subcarrier signals can
be separated by a simple filter. More specifically, the frequency interval
between the two pilot carrier signals and that between the subcarrier
signals are made equal.
For three or more systems of subcarrier signals to be transmitted as radio
signals, only two pilot carrier signals f.sub.LO are prepared. Each of the
three or more systems of subcarrier signals is converted into a radio
signal having a desired carrier frequency. Since transmission and
reception systems have the same arrangement, the down link signal
processing system will be described in the sixth embodiment for the
descriptive convenience.
FIG. 15 is a block diagram showing the arrangement of the sixth embodiment.
Transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p are arranged in
predetermined service areas. A transmitting/receiving station 10B manages
and operates the transmitting/receiving devices 32B-1, 32B-2, . . . ,
32B-p and supplies intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp to be transmitted from the
transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p, which are
synthesized with two pilot carrier signals f.sub.LO1 and f.sub.LO2 to the
corresponding transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p.
The transmitting/receiving station 10B has the E/O converter 18 formed from
a laser element 86 and a laser driver 84, the adder 16, frequency
converters 88-1, 88-2, . . . , 88-p, modulators 12-1, 12-2, . . . , 12-p,
and first and second pilot carrier generators 14-1 and 14-2.
The modulators 12-1, 12-2, . . . , 12-p output signals modulated with input
data to the frequency converters 88-1, 88-2, . . . , 88-p, respectively.
The frequency converters 88-1, 88-2, . . . , 88-p frequency-converts the
modulated input signals and output the signals.
The first and second pilot carrier generators 14-1 and 14-2 are circuits
for generating the pilot carrier signals f.sub.LO1 and f.sub.LO2 having
different frequencies. The adder 16 synthesizes the two pilot carrier
signals f.sub.LO1 and f.sub.LO2 with the outputs f.sub.IF1, f.sub.IF2,
f.sub.IFp from the frequency converters 88-1, 88-2, . . . , 88-p. The
laser driver 84 drives the laser element 86 in accordance with the signal
synthesized by the adder 16. The laser element 86 is caused by the laser
driver 84 to output a laser beam optically modulated in accordance with
the synthesized signal from the adder 16 and send the signal to the
optical fiber 30.
Each of the transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p
comprises the O/E converter 34, bandpass filter 38 for separating the
intermediate frequency subcarrier signal f.sub.IF1, bandpass filters 40-1
and 40-2 for separating the pilot carrier signals f.sub.LO1 and f.sub.LO2,
a frequency converter 82 formed from a multiplier and a power amplifier,
and the antenna 48.
The O/E converter 34 converts an optical signal. sent through the optical
fiber 30 into an electrical signal. The bandpass filter 38 separates the
intermediate frequency subcarrier signal from the electrical signal. The
bandpass filters 40-1 and 40-2 separate the first and second pilot carrier
signals from the electrical signal from the O/E converter 34.
The frequency converter 82 frequency-converts the separated first and
second pilot carrier signals and intermediate frequency subcarrier signal
and sends a data signal to the antenna 48.
The optical fiber 30 is an optical transmission line connecting the
transmitting/receiving station 10B to the transmitting/receiving devices
32B-1, 32B-2, . . . , 32B-p and has the optical divider 34 inserted in the
midway. The optical divider 34 divides the optical signal from the laser
element 86 and distributes the optical signals to all the
transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p connected.
In this system, the intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp optically transmitted from the
transmitting/receiving station 10B to the transmitting/receiving devices
32B-1, 32B-2, . . . , 32B-p are subcarrier-multiplexed at a large
frequency interval, as shown in FIG. 16, such that the signals can be
separated by a simple filter.
In addition to the intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp, the pilot carrier signals f.sub.LO1 and
f.sub.LO2 are transmitted. In this system, the number of pilot carrier
signals f.sub.LO1 and f.sub.LO2 is always two independently of the number
of intermediate frequency subcarrier signals. The frequencies of the two
pilot carrier signals f.sub.LO1 and f.sub.LO2 are set such that a
frequency to be sent from the antenna is obtained when integral multiples
of the frequencies f.sub.LO1 and f.sub.LO2 of the pilot carrier signals
are appropriately added/subtracted to/from the frequencies of the
intermediate frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . ,
f.sub.IFp.
In the system having the above arrangement, in the transmitting/receiving
station 10B, data to be transmitted to the transmitting/receiving devices
32B-1, 32B-2, . . . , 32B-p are supplied to the modulators 12-1, 12-2, . .
. , 12-p corresponding to the transmitting/receiving devices and modulated
into an appropriate form by QPSK, QAM, or the like.
Outputs from the modulators 12-1, 12-2, . . . , 12-p are supplied to the
frequency converters 88-1, 88-2, . . . , 88-p, respectively, and converted
into appropriate intermediate frequency subcarrier frequencies f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp corresponding to the transmitting/receiving
devices 32B-1, 32B-2, . . . , 32B-p.
At this time, the frequencies of the plurality of intermediate frequency
subcarrier signals are sufficiently separated from each other, as shown in
FIG. 16. For example, the bandwidth of each intermediate frequency
subcarrier signal is about 20 [MHz], an interval of about 100 [MHZ] is
set.
The first and second pilot carrier generators 14-1 and 14-2 generate the
pilot carrier signals f.sub.LO1 and f.sub.LO2 having different
frequencies, respectively. These pilot carrier signals f.sub.LO1 and
f.sub.LO2 are input to the adder 16.
The adder 16 multiplexes the intermediate frequency subcarrier signals
f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp output from the frequency
converters 88-1, 88-2, . . . , 88-p with the first and second pilot
carrier signals f.sub.LO1 and f.sub.LO2 generated by the first and second
pilot carrier generators 14-1 and 14-2, respectively. The multiplexed
signal is input to the laser driver 84 and converted into an optical
signal by the laser element 86.
The optical signal converted by the laser element 86 and output is input to
the optical fiber 30. The optical divider 34 is inserted midway along the
optical fiber 30, so the optical signal from the laser element 86 is
divided and distributed to all the transmitting/receiving devices 32B-1,
32B-2, . . . , 32B-p connected.
The optical signal transmitted to a transmitting/receiving device, e.g.,
the transmitting/receiving device 32B-1 is converted into an electrical
signal by the O/E converter 34. The intermediate frequency subcarrier
signal f.sub.IF1 that is sent to the self station is separated from the
obtained electrical signal by the bandpass filter 38. The bandpass filter
38 can be formed from a simple filter having a relatively small Q value.
The bandpass filters 40-1 and 40-2 further extract the first pilot carrier
signal f.sub.LO1 and second pilot carrier signal f.sub.LO2 from the signal
obtained by the O/E converter 34, respectively. The bandpass filters 40-1
and 40-2 can also be constructed by simple filters having relatively small
Q values.
The intermediate frequency subcarrier signal f.sub.IF1 separated by the
bandpass filter 38 and the two pilot carrier signals f.sub.LO1 and
f.sub.LO2 separated by the bandpass filters 40-1 and 40-2 are input to the
frequency converter 82. The frequency converter 82 appropriately converts
the intermediate frequency subcarrier signal f.sub.IF1 into a target radio
frequency by appropriately multiplying, adding, and subtracting the
frequencies of these three signals.
The frequency converter 82 mainly comprises a mixer, a multiplier, a
filter, a switch, and the like. Details of the arrangement will be
described later.
The intermediate frequency subcarrier signal f.sub.IF1 converted into a
desired radio frequency by the frequency converter 82 is radiated from the
antenna 48 of the self station into air and sent to a terminal in the
service area of the self station.
According to the sixth embodiment, when data signals are to be optically
transmitted from the transmitting/receiving station 10B to the plurality
of transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p as
subcarriers, only two pilot carrier signals f.sub.LO1 and f.sub.LO2 are
used while setting a large frequency interval between the intermediate
frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp, and
the frequencies are set in advance such that a frequency to be sent from
the antenna is obtained when integral multiples of the frequencies of the
pilot carrier signals f.sub.LO1 and f.sub.LO2 are appropriately
added/subtracted to/from the frequencies of the intermediate frequency
subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp.
Since the frequency interval between the intermediate frequency subcarrier
signals is set to be sufficiently large, each transmitting/receiving
device can extract the intermediate frequency subcarrier signal addressed
to the self station from the subcarrier-multiplexed data signal using a
simple filter.
The frequencies of the pilot carrier signals f.sub.LO1 and f.sub.LO2 are
set to satisfy the above relationship. Therefore, the advantage in use of
the pilot carrier signals f.sub.LO1 and f.sub.LO2 can be maintained: the
signal to be sent from the antenna has high frequency stability although
only two pilot carrier signals f.sub.LO1 and f.sub.LO2 are used.
Additionally, since the number of pilot carrier signals f.sub.LO1 and
f.sub.LO2 is as small as two, high-quality transmission can be performed
without largely decreasing the optical modulation index of the
intermediate frequency subcarrier signal in optical transmission.
In the present invention, the intermediate frequency subcarrier signals to
be optically transmitted from the transmitting/receiving station to the
plurality of transmitting/receiving devices are subcarrier-multiplexed at
a large frequency interval such that the signals can be separated by a
simple filter.
In addition to the intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp, the pilot carrier signals f.sub.LO1 and
f.sub.LO2 are transmitted from the transmitting/receiving station 10B to
the plurality of transmitting/receiving devices 32B-1, 32B-2, . . . ,
32B-p. Unlike the prior art, the number of pilot carrier signals f.sub.LO1
and f.sub.LO2 is only two independently of the number of intermediate
frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp. The
frequencies of the two pilot carrier signals f.sub.LO1 and f.sub.LO2 and
the frequencies of the intermediate frequency subcarrier signals
f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp are set such that the frequency of
each intermediate frequency subcarrier signal can be up-converted into a
frequency to be sent from the antenna when integral multiples of the
frequencies of the two pilot carrier signals f.sub.LO1 and f.sub.LO2 are
appropriately added/subtracted to/from the frequencies of the intermediate
frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp. Note
that "integers" for "integral multiples" include "0" and all positive and
negative integers.
In the frequency converter 82, the pilot carrier signals f.sub.LO1 and
f.sub.LO2 are multiplied by a multiplier or mixer. Addition/subtraction of
signals or the frequencies of multiplied pilot carrier signals is also
performed using the mixer.
With this arrangement, even when three or more transmitting/receiving
devices are accommodated in a PON, only two pilot carrier signals
f.sub.LO1 and f.sub.LO2 are necessary to be sent to all the
transmitting/receiving devices. In addition, the frequency interval
between the intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp can be set to be sufficiently large such that
each signal can be extracted using a simple filter.
As a consequence, the optical modulation index of the intermediate
frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp is
not sacrificed by a number of pilot carrier signals, and satisfactory
transmission can be performed. In addition, the process of extracting a
necessary signal after reception of the optical signal becomes simple and
inexpensive.
Next, a specific example of the frequency arrangement of the two pilot
carrier signals f.sub.LO1 and f.sub.LO2 will be described as a seventh
embodiment.
Seventh Embodiment
FIG. 17 is a graph showing a specific frequency arrangement of two pilot
carrier signals f.sub.LO1 and f.sub.LO2. As shown in FIG. 17, modulated
data signals to be transmitted to transmitting/receiving devices 32B-1,
32B-2, . . . , 32B-p are subcarrier-multiplexed with center frequencies
f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp. The frequency difference between
adjacent intermediate frequency subcarrier signals is .DELTA.F or an
integral multiple of .DELTA.F.
The frequency difference between the pilot carrier signals f.sub.LO1 and
f.sub.LO2 is also .DELTA.F. In the seventh embodiment, as shown in FIG.
17, the pilot carrier signals f.sub.LO1 and f.sub.LO2 are arranged in a
frequency region higher than that range where the intermediate frequency
subcarrier signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp are arranged.
The operation of the present invention will be described using specific
numerical data.
Assume that four intermediate frequency subcarrier signals f.sub.IF1,
f.sub.IF2, f.sub.IF3, f.sub.IF4 having frequencies of 100 [MHz], 200
[MHz], 400 [MHz], and 500 [MHz], respectively, are subcarrier-multiplexed,
and the first and second pilot carrier signals f.sub.LO1 and f.sub.LO2
have frequencies of 2 [GHz] and 1.9 [GHz], respectively. A radio frequency
F.sub.0 to be sent from an antenna 48 of the transmitting/receiving device
32B is 22 [GHz].
In the transmitting/receiving device 32B-1 which uses the intermediate
frequency subcarrier signal f.sub.IF1, the bandpass filter 38 extracts the
intermediate frequency subcarrier signal f.sub.IF1 from the
subcarrier-multiplexed optical signal, and the bandpass filters 40-1 and
40-2 extract the first and second pilot carrier signals f.sub.LO1 and
f.sub.LO2, respectively.
To up-convert the intermediate frequency subcarrier signal f.sub.IF1 (=100
[MHZ]) into the radio frequency F.sub.0 (=22 [GHz]) using these signals,
the second pilot carrier signal f.sub.LO2 (=1.9 [GHz]) is added to a
frequency (=20 [GHz]) obtained by multiplying the first pilot carrier
signal f.sub.LO1 by 10, and the frequency f.sub.IF1 (=100 [MHz]) of the
intermediate frequency subcarrier signal is added to the resultant
frequency. That is,
f.sub.LO1 (=2 [GHz]).times.10+f.sub.LO2 (=1.9 [GHz])+f.sub.IF1 (=100
[MHz])=20+1.9+0.1=22 [GHz]
As a result, the intermediate frequency subcarrier signal f.sub.IF1 having
a frequency of 100 [MHz] can be up-converted into the radio frequency
F.sub.0 of 22 [GHz] using the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2.
To up-convert the intermediate frequency subcarrier signal f.sub.IF2 (=200
[MHz]) into the radio frequency F.sub.0, a frequency (=3.8 [GHz]) obtained
by multiplying the second pilot carrier signal f.sub.LO2 by 2 is added to
a frequency (=18 [GHz]) obtained by multiplying the first pilot carrier
signal f.sub.LO1 by 9, and the intermediate frequency subcarrier signal
f.sub.IF2 (=200 [MHz]) is added to the resultant frequency. That is
f.sub.LO1 (=2 [GHz]).times.9+f.sub.LO2 (=1.9 [GHz]).times.2+f.sub.IF2 (=200
[MHz])=18+3.8+0.2=22 [GHz]
As a result, the intermediate frequency subcarrier signal f.sub.IF2 having
a frequency of 200 [MHz] can be up-converted into the radio frequency
F.sub.0 of 22 [GHz] using the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2.
To up-convert the intermediate frequency subcarrier signal f.sub.IF3 (=400
[MHz]) into the radio frequency F.sub.0, a frequency (=7.6 [GHz]) obtained
by multiplying the second pilot carrier signal f.sub.LO2 by 4 is added to
a frequency (=14 [GHz]) obtained by multiplying the first pilot carrier
signal f.sub.LO1 by 7, and the intermediate frequency subcarrier signal
f.sub.IF3 (=400 [MHz]) is added to the resultant frequency. That is
f.sub.LO1 (=2 [GHz]).times.7+f.sub.LO2 (=1.9 [GHz]).times.4+f.sub.IF3 (=400
[MHz])=14+7.6+0.4=22 [GHz]
As a result, the intermediate frequency subcarrier signal f.sub.IF3 having
a frequency of 400 [MHz] can be up-converted into the radio frequency
F.sub.0 of 22 [GHz] using the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2.
To up-convert the intermediate frequency subcarrier signal f.sub.IF4 (=500
[MHz]) into the radio frequency F.sub.0, a frequency (=9.5 [GHz]) obtained
by multiplying the second pilot carrier signal f.sub.LO2 by 5 is added to
a frequency (=12 [GHz]) obtained by multiplying the first pilot carrier
signal f.sub.LO1 by 6, and the frequency f.sub.IF4 (=500 [MHz]) of the
intermediate frequency subcarrier signal is added to the resultant
frequency. That is
f.sub.LO1 (=2 [GHz]).times.6+f.sub.LO2 (=1.9 [GHz]).times.5+f.sub.IF4 (=500
[MHz])=12+9.5+0.5=22 [GHz]
As a result, the intermediate frequency subcarrier signal f.sub.IF4 having
a frequency of 500 [MHz] can be up-converted into the radio frequency
F.sub.0 of 22 [GHz] using the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2.
As described above, using only the two pilot carrier signals F.sub.LO1 and
f.sub.LO2, the intermediate frequency subcarrier signals can be
up-converted into the radio frequency F.sub.0 in all
transmitting/receiving devices.
Arrangements of the frequency converter 82 will be described next with
reference to FIGS. 18, 19, and 22.
[First Arrangement of Frequency Converter]
FIG. 18 shows a first arrangement of the frequency converter 82 in the
transmitting/receiving device.
The frequency converter 82 shown in FIG. 18 comprises multipliers 92 and
94, mixers 96 and 98, and bandpass filters 100 and 102.
The multiplier 92 multiplies the first pilot carrier signal f.sub.LO1 by
.vertline.n.vertline. and supplies the signal to the mixer 96. The mixer
96 also receives the intermediate frequency subcarrier signal f.sub.IF and
mixes this signal with the signal from the multiplier 92, which is
multiplied by .vertline.n.vertline.. The bandpass filter 100 extracts a
desired frequency component from the signal from the mixer 96.
The multiplier 94 multiplies the second pilot carrier signal f.sub.LO2 by
.vertline.m.vertline. and supplies the signal to the mixer 98. The mixer
98 also receives the signal from the bandpass filter 100 and mixes this
signal with the second pilot carrier signal f.sub.LO2 from the multiplier
94, which is multiplied by .vertline.m.vertline.. The bandpass filter 102
extracts a desired frequency component from the signal from the mixer 98.
In the frequency converter 82 having the arrangement shown in FIG. 18, the
first pilot carrier signal f.sub.LO1 is multiplied by a necessary
multiplying factor (.vertline.n.vertline.) by the multiplier 92. In the
example of the above-described intermediate frequency subcarrier signal
f.sub.IF1, the first pilot carrier signal f.sub.LO1 is multiplied by 10
and the second pilot carrier signal f.sub.LO2 is multiplied by a necessary
multiplying factor (.vertline.m.vertline.) by the multiplier 94. In the
above example, the second pilot carrier signal f.sub.LO2 is multiplied by
1, i.e., passes through the multiplier 94 without any multiplication. The
intermediate frequency subcarrier signal f.sub.IF1 (100 [MHz]) is mixed
with the first pilot carrier signal f.sub.LO1 (20 [GHz]) multiplied by
.vertline.n.vertline. by the mixer 96. Of the sum frequency component
(20.1 [GHz]) and difference frequency component (19.9 [GHz]) output from
the mixer 96, the sum frequency component (20.1 [GHz]) is selected by the
filter 100 and outputted.
The output from the filter 100 is mixed with the second pilot carrier
signal f.sub.LO2 (1.9 [GHz]) multiplied by .vertline.m.vertline. by the
mixer 98. Of the sum frequency component (22 [GHz]) and difference
frequency component (18.2 [GHz]) output from the mixer 98, the sum
frequency component (22 [GHz]) is selected by the bandpass filter 102 and
outputted.
In this way, the frequency converter 82 can obtain the intermediate
frequency subcarrier signal up-converted to the target frequency.
[Second Arrangement of Frequency Converter]
FIG. 19 shows another arrangement of the frequency converter 82 in the
transmitting/receiving device.
The frequency converter 82 shown in FIG. 19 comprises the multipliers 92
and 94, mixers 96 and 98, and bandpass filters 100 and 102.
The multiplier 92 multiplies the first pilot carrier signal f.sub.LO1 by
.vertline.n.vertline. and supplies the signal to the mixer 96. The
multiplier 94 multiplies the second pilot carrier signal f.sub.LO2 by
.vertline.m.vertline. and supplies the signal to the mixer 96. The mixer
96 mixes the multiplied outputs from the multipliers 92 and 94.
The bandpass filter 100 extracts a desired frequency component from the
signal from the mixer 96 and outputs the frequency component to the mixer
98. The mixer 98 also receives the intermediate frequency subcarrier
signal f.sub.IF, mixes this signal with the signal passed through the
bandpass filter 100, and outputs the mixed signal to the filter 102. The
bandpass filter 102 extracts a desired frequency component from the signal
from the mixer 98.
In the frequency converter 82 having the arrangement shown in FIG. 19, the
first pilot carrier signal f.sub.LO1 is multiplied by a necessary
multiplying factor (.vertline.n.vertline.) by the multiplier 92. In the
example of the above-described intermediate frequency subcarrier signal
f.sub.IF1, the first pilot carrier signal f.sub.LO1 is multiplied by 10
and the second pilot carrier signal f.sub.LO2 is multiplied by a necessary
multiplying factor (.vertline.m.vertline.) by the multiplier 94. In the
above example, the second pilot carrier signal f.sub.LO2 is multiplied by
1, i.e., passes through the multiplier 94 without any multiplication. The
first pilot carrier signal f.sub.LO1 (20 [GHz]) multiplied by
.vertline.n.vertline. and second pilot carrier signal f.sub.LO2 (1.9
[GHz]) multiplied by .vertline.m.vertline. are mixed by the mixer 96, and
a sum frequency component (21.9 [GHz]) and difference frequency component
(18.1 [GHz]) are output.
Of these frequency components, the sum frequency component (21.9 [GHz]) is
selected by the bandpass filter 100 and outputted. The intermediate
frequency subcarrier signal f.sub.IF1 (100 [MHz]) and output from the
bandpass filter 100 are mixed by the mixer 98. Of the sum frequency
component (22 [GHz]) and difference frequency component (21.8 [GHz])
output from the mixer 98, the sum frequency component (22 [GHz]) is
selected by the filter 102 and outputted.
In this manner, the frequency converter 82 can obtain the intermediate
frequency subcarrier signal up-converted to the target frequency (22
[GHz]).
[Arrangement of Multiplier]
As the multipliers 92 and 94 used for the frequency converter 82,
conventional frequency multipliers with fixed multiplying factors are used
if the multiplying factors .vertline.n.vertline. and .vertline.m.vertline.
can be fixed. However, the multiplying factors may change sometimes
depending on the system arrangement. That is, the frequency of an
intermediate frequency subcarrier signal sent to the self station may
change. In such a case, the multipliers 92 and 94 are constructed as shown
in FIG. 20. In this example, the multiplying factor can be changed from 1
(without any multiplication) to k.
An input is supplied to a divider 114 through a switch 110-1. The switch
110-1 is a path change-over switch for selectively supplying the input to
the divider 114 side or a selector 112 side. When the path is switched to
the selector 112 side, the input signal is output without any
multiplication.
The divider 114 distributes the input signal to k paths (outputs). The
first and second outputs are input to a mixer 116-1. The mixer 116-1
outputs the sum frequency component and difference frequency component of
the two input signals. From the output from the mixer 116-1, the sum
frequency component is extracted by a bandpass filter 118-1, and a signal
multiplied by 2 is output.
The output from the bandpass filter 118-1 is supplied to a switch 110-2.
The switch 110-2 is a path change-over switch for supplying the signal to
a mixer 116-2 of the next stage or the selector 112 side. The mixer 116-2
mixes the output from the bandpass filter 118-1 with the output from the
divider 114 and outputs the mixed signal. The mixer 116-2 mixes the output
from the mixer 116-1 for multiplication by 2 with the output from the
divider 114, i.e., the original frequency signal. Hence, frequency up
conversion of multiplication by 3 is performed. A bandpass filter 118-2
extracts the sum frequency component from the output from the mixer 116-2.
In a similar manner, an output from a bandpass filter 118-i is supplied to
a switch 110-(i+1). The switch 110-(i+1) is a path change-over switch for
supplying this signal to a mixer 116-(i+1) or the selector 112 side. The
mixer 116-(i+1) mixes the output from the bandpass filter 118-i with the
output from the divider 114 and outputs the mixed signal. The output from
the mixer 116-(i+1) is supplied to a bandpass filter 118-(i+1) for
extracting the sum frequency component, and frequency up conversion of
(i+1) multiplication is performed.
The selector 112 selects one of the signal (original signal) from the
switch 110-1 and signals frequency-up-converted by the respective stages
and outputs the selected signal.
In the example shown in FIG. 20, multiplication by 1 (without any
multiplication) to k can be performed. The input pilot carrier signals
f.sub.LO1 and f.sub.LO2 are input to the switch 110-1. When the signals
are to be multiplied by 1, i.e., output without any multiplication, the
switch 110-1 is switched to the selector 112 side, and the selector 112 is
switched to the switch 110-1 side.
In this way, a carrier signal multiplied by 1 is output. For multiplication
by 2 or more, the switch 110-1 is switched and connected to the divider
114 side.
The divider 114 divides the input signal into k components. Two components
are input to the two terminals of the mixer 116-1. The sum frequency
component generated by the mixer 116-1 is selected by the bandpass filter
118-1 and outputted.
The output from the bandpass filter 118-1 is connected to the switch 110-2.
When the switch 110-2 is connected to the selector 112 side, a signal
multiplied by 2 is output. When the switch 110-2 is connected to the mixer
116-2 on the output side, the signal is multiplied by 3 or more.
Similarly, the switches 110-2, 110-3, . . . and selector 112 are
controlled such that the mixers and filters are alternately connected, and
the signal multiplied by a necessary multiplying factor is connected to
the output terminal.
With this arrangement, a multiplier with a variable multiplying factor can
be constructed.
Another arrangement may be employed for the multiplier with a variable
multiplying factor. FIG. 21 shows a multiplier 92 or 94 having a nonlinear
element such as a diode 119 to which an input signal is supplied, a filter
bank formed of filters 118-1, 118-2, . . . , 118-k to which an output
signal of the diode 119 is supplied via a divider 114, and a selector 112
selecting one of output signals from the filters 118-1, 118-2, . . . ,
118-k.
Each of filters 118-1, 118-2, . . . , 118-k of the filter bank has pass
characteristics corresponding to its harmonics. A filter corresponding to
a desired multiplying factor is selected from the filter bank, thereby
constructing a multiplier with a variable multiplying factor. Each output
from the filter bank 118 is supplied to a selector 112. The selector 112
is controlled to select any path from the filter bank 118 and the signal
multiplied by a necessary multiplying factor is connected to the output
terminal.
[Third Arrangement of Frequency Converter]
Still another arrangement of the frequency converter 82 with a variable
multiplying factor will be described with reference to FIG. 22.
The first pilot carrier signal f.sub.LO1 is distributed into (n+m)
components by a divider 128-1. The second pilot carrier signal f.sub.LO2
is distributed into (n+m) by a divider 128-2.
Switches 126-1, 126-2, . . . , 126-(n+m) select outputs from the divider
128-1 or 128-2. Outputs from the switches 126-1 and 126-2 are supplied to
a mixer 122-1. An output from the mixer 122-1 is supplied to a mixer 122-2
through a bandpass filter 124-1.
In a similar manner, a mixer 122-i mixes an output from a bandpass filter
124-(i-1) with an output from a switch 126-(i+1). A mixer 122-(n+m) at the
final stage mixes an output from a bandpass filter 124-(n+m-1) with the
intermediate frequency subcarrier signal f.sub.IF. An output from the
mixer 122-(m+n) is output through a bandpass filter 124-(n+m). Even when
the values n and m change, the value (n+m) does not change.
In this arrangement, the two pilot carrier signals f.sub.LO1 and f.sub.LO2
are distributed, by the dividers 128-1 and 128-2, into plural signals
whose number equals a maximum multiplying factor. The mixer 122-1 has two
terminals to which one of the signals f.sub.LO1, f.sub.LO2, and f.sub.IF
is directly input, and each of the mixers 122-2 to 122-(n+m-1) has one
terminal to which one of the signals f.sub.LO1, f.sub.LO2, and f.sub.IF is
directly input. Hence, a total of n+m input terminals are present.
The switches 126-1, 126-2, . . . are controlled such that the first pilot
carrier signal f.sub.LO1 is input to n terminals and the second pilot
carrier signal f.sub.LO2 is input to m terminals. The filters 124-1,
124-2, . . . connected to the output sides of the mixers 122-1, 122-2, . .
. select sum frequency components from the difference frequency components
and sum frequency components between the mixed signals output from the
122-1, 122-2, . . . and the directly input signals f.sub.LO1, f.sub.LO2,
and f.sub.IF and output the sum frequency components.
In this arrangement, the filter 124-(n+m-1) outputs a sum carrier of the
first pilot carrier signal f.sub.LO1 multiplied by n and the second pilot
carrier signal f.sub.LO2 multiplied by m. This signal is mixed with the
intermediate frequency subcarrier signal f.sub.IF by the mixer 122-(n+m).
As a result, the sum frequency component and difference frequency
component are output from the mixer 122-(n+m), and the sum frequency
component is selected by the bandpass filter 124-(m+n) and output, so a
multiplied signal f.sub.LO1.times.n+f.sub.LO2.times.m+f.sub.IF is
obtained.
With this arrangement, a frequency converter whose multiplying factor can
be changed to a desired value can be constructed.
[Fourth Arrangement of Frequency Converter]
Still another arrangement of the frequency converter 82 will be described.
In the above examples, n and m are "0" or positive integers.
However, in the present invention, n and m can be negative values. For
example, assume that a relation
10.times.f.sub.LO1 +f.sub.Ifi =F.sub.0
holds between the target frequency F.sub.0, pilot carrier signals f.sub.LO1
and f.sub.LO2 (=f.sub.LO1 -.DELTA.f), and intermediate frequency
subcarrier signal f.sub.IF1.
For an intermediate frequency subcarrier signal f.sub.IFi+1 (=f.sub.IFi
+.DELTA.F),
9.times.f.sub.LO1 +f.sub.LO2 +f.sub.Ifi =F.sub.0
That is, the target frequency F.sub.0 can be synthesized using positive
values for both n and m, i.e., n=9 and m=1.
On the other hand, for an intermediate frequency subcarrier signal having a
frequency of f.sub.IFi-1 (=f.sub.IFi -.DELTA.F),
40.times.f.sub.LO1 -f.sub.LO2 +f.sub.Ifi-1 =F.sub.0
That is, one of n and m is set to be negative: n=40 and m=-1.
In the arrangement of the frequency converter 82 shown in, e.g., FIG. 18,
frequency synthesis using a negative value, i.e., subtraction can be
executed by selecting, by the bandpass filter 102, the difference
frequency component from the sum frequency component and difference
frequency component generated by the mixer 98 and outputting the
difference frequency component.
When negative values can be used as n and m, the upper and lower limits of
the intermediate frequency subcarrier signal, i.e., limitation on the
number of channels of subcarrier multiplex is moderated, and a flexible
system can be constructed.
Various arrangements of the frequency converter have been described above.
A pilot carrier signal separator will be described next.
[Arrangement of Pilot Carrier Signal Separator]
The pilot carrier signal separators (bandpass filters 40-1 and 40-2 in FIG.
15) in each of the transmitting/receiving devices 32B-1, 32B-2, . . . ,
32B-p can be realized by filters having small Q values, as described
above. However, when PLLs (Phase-Locked Loops) are used together with the
filters, the pilot carrier signals f.sub.LO1 and f.sub.LO2 with higher
quality can be separated.
An optical signal sent from the transmitting/receiving station 10B contains
not only the necessary intermediate frequency subcarrier signals f.sub.IF
and pilot carrier signals f.sub.LO1 and f.sub.LO2 but also various noise
components.
There are noise called relative intensify noise originally contained in the
optical signal, thermal noise generated by the optical receiver, and shot
noise generated when a photocurrent flows to the photodiode. These noise
components are generally white noise.
When the pilot carrier signals f.sub.LO1 and f.sub.LO2 are separated from
the optical signal containing such white noise using only filters with
small Q values as the bandpass filters 40-1 and 40-2, a number of noise
components are also extracted.
The demand for the noise amount changes depending on the system. Some
systems can directly use components extracted by filters. However, in a
system with a strict demand for noise, a PLL is connected to the output
side of a filter having a small Q value. With this arrangement, the
carrier-to-noise ratios of the pilot carrier signals f.sub.LO1 and
f.sub.LO2 can be made high.
As mentioned in the description of prior arts, in some systems, the center
frequency of a radio signal radiated from the antenna 48 of the self
station slightly changes in units of the plurality of
transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p.
For example, when radio waves in the same 2 [GHz] band are radiated, the
center frequency of the radio signal radiated from one
transmitting/receiving device is 2.000000 [GHz], and the center frequency
of the radio signal radiated from another transmitting/receiving device is
2.000100 [GHz]. That is, the center frequency changes at an interval of,
e.g., 100 [kHz].
In this case as well, in the present invention, the frequency interval
between subcarrier signals to be optically transmitted is set to be much
larger than the frequency interval (e.g., 100 [kHz]) of the radio range.
As a result, processing such as data separation in the
transmitting/receiving device is facilitated.
When the present invention is applied to a system in which the frequencies
of radio signals radiated from the transmitting/receiving devices 32B-1,
32B-2, . . . , 32B-p are slightly different from each other, a frequency
difference corresponding to the frequency difference (e.g., 100 [kHz])
between the radio waves of the transmitting/receiving devices 32B-1,
32B-2, . . . , 32B-p is applied to the frequencies of subcarrier signals
to be optically transmitted in advance as an offset.
Since the frequency interval between the subcarrier signals to be optically
transmitted is very large, this offset does not affect the operation of
the present invention at all.
An example will be described below.
Assume that a signal radiated from the self station antenna 48 of the
transmitting/receiving device 32B-1 has a center frequency F.sub.01, and a
signal radiated from the self-station antenna 48 of the
transmitting/receiving device 32B-2 has a center frequency F.sub.02
(=F.sub.01 +.DELTA.F.sub.R).
The center frequency of the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2 to be optically transmitted is f.sub.LO2
(=f.sub.LO1 +.DELTA.F). The intermediate frequency subcarrier signal
f.sub.IF1 is subcarrier-transmitted with a center frequency F.sub.01 that
is given by F.sub.01 =7.times.f.sub.LO1 +3.times.f.sub.LO2 +f.sub.IF1.
The intermediate frequency subcarrier signal f.sub.IF2 is
subcarrier-transmitted with a center frequency F.sub.02 that is given by
F.sub.02 =8.times.f.sub.LO1 +2.times.f.sub.LO2 +f.sub.IF2.
At this time, the intermediate frequency subcarrier signals f.sub.IF1 and
f.sub.IF2 are determined such that the difference between the center
frequencies f.sub.IF1 and f.sub.IF2 of the two intermediate frequency
subcarrier signals is represented by
.DELTA.F+.DELTA.F.sub.R.
Since
.DELTA.F>>.DELTA.F.sub.R
then
.DELTA.F+.DELTA.F.sub.R =.DELTA.F.
In this case, .DELTA.F.sub.R is the above-described offset amount of the
subcarrier frequency.
In the embodiments of the present invention, such a small offset has not
been specified so far and will not particularly be described in examples
later. However, note that the method described in the seventh embodiment
may be employed in practicing the present invention.
Eighth Embodiment
FIG. 23 is a graph showing the frequency arrangement in optical
transmission of the eighth embodiment. Intermediate frequency subcarrier
signals are subcarrier-multiplexed to center frequencies f.sub.IF1,
f.sub.IF2, . . . , f.sub.IFp. The center frequency interval between
adjacent intermediate frequency subcarrier signals is an integral multiple
(.gtoreq.1) of .DELTA.F. A first pilot carrier signal f.sub.LO1 is set in
a frequency band higher than that of the intermediate frequency subcarrier
signals f.sub.IF1, f.sub.IF2, . . . , f.sub.IFp, as in the case described
in the seventh embodiment. On the other hand, a second pilot carrier
signal f.sub.LO2 is set in a frequency band lower than that of the
intermediate frequency subcarrier signals f.sub.IF1, f.sub.IF2, . . . ,
f.sub.IFp and has the frequency .DELTA.F in this embodiment.
Assume that for the intermediate frequency subcarrier signal f.sub.IF1, the
frequency F.sub.0 of a signal to be radiated from the antenna is F.sub.0
=n.times.f.sub.LO1 +f.sub.IF1. The intermediate frequency subcarrier
signal f.sub.IF2 (=f.sub.IF1 +.DELTA.F) is synthesized such that the
frequency F.sub.0 becomes F.sub.0 =n.times.f.sub.LO1 -f.sub.LO2
+f.sub.IF2. That is, the intermediate frequency subcarrier signal
f.sub.IF1 is synthesized while setting a multiplying factor m of the
second pilot carrier signal f.sub.LO2 at 0, and the intermediate frequency
subcarrier signal f.sub.IF2 is synthesized while setting the multiplying
factor m at -1. The intermediate frequency subcarrier signals f.sub.IF3,
f.sub.IF4, . . . , f.sub.IFp are also synthesized in a similar manner.
If a negative multiplying factor m is not preferable, for the intermediate
frequency subcarrier signal f.sub.IFp having the highest frequency,
F.sub.0 =n.times.f.sub.LO1 +f.sub.IFp is set. For an intermediate
frequency subcarrier signal f.sub.IFp-1 (=f.sub.IFp -.DELTA.F), F.sub.0
=n.times.f.sub.LO1 +f.sub.LO2 +f.sub.IFp-1 is set. Frequencies are
sequentially synthesized in a similar manner.
For an intermediate frequency subcarrier signal f.sub.IFi, F.sub.0
=n.times.f.sub.LO1 +f.sub.IFi is set. For an intermediate frequency
subcarrier signal having a frequency lower than f.sub.IFi, an integral
multiple of f.sub.LO2 is appropriately added. For an intermediate
frequency subcarrier signal having a frequency higher than f.sub.IFi, an
integral multiple of f.sub.LO2 is appropriately subtracted.
The arrangement of the system to which the eighth embodiment is applied is
the same as in FIG. 15. The arrangement of a frequency converter 82 in
each of the transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p is
the same as in FIGS. 18 or 19.
Although the arrangement is the same as in the eighth embodiment, the
operation is slightly different. The operation of the arrangement shown in
FIG. 18 will be described, assuming that the intermediate frequency
subcarrier signal used by the station is f.sub.IF3 (=f.sub.IF1
+2.times..DELTA.F), and the target center frequency of frequency
conversion is F.sub.0 =n.times.f.sub.LO1 +f.sub.IF1.
Referring to FIG. 18, the intermediate frequency subcarrier signal
f.sub.IF3 is input to the mixer 96. On the other hand, the first pilot
carrier signal f.sub.LO1 is input and multiplied by n (>0) by the
multiplier 92. Unlike the arrangement in FIG. 18, the value n is constant
independently of the center frequency in subcarrier transmission of the
intermediate frequency subcarrier signal.
The intermediate frequency subcarrier signal f.sub.IF3 and the first pilot
carrier signal f.sub.LO1 multiplied by n are mixed by the mixer 96, so a
sum frequency component (n.times.f.sub.LO1 +f.sub.IF3) and difference
frequency component (n.times.f.sub.LO1 -f.sub.IF3) are obtained. Of the
sum frequency component (n.times.f.sub.LO1 +f.sub.IF3) and difference
frequency component (n.times.f.sub.LO1 -f.sub.IF3) output from the mixer
96, the sum frequency component is selected by the bandpass filter 100 and
outputted.
The second pilot carrier signal f.sub.LO2 is multiplied by
.vertline.m.vertline. by the multiplier 94. In this example, since the
multiplying factor m is -2, the second pilot carrier signal f.sub.LO2 is
multiplied by 2 by the multiplier 94.
The output from the bandpass filter 100 and the output from the multiplier
94 are mixed by the mixer 98. Consequently, the sum frequency component
(n.times.f.sub.LO1 +f.sub.IF3 +.vertline.m.vertline..times.f.sub.LO2) and
difference frequency component (n.times.f.sub.LO1 +f.sub.IF3
-.vertline.m.vertline..times.f.sub.LO2) between the two signals are output
from the mixer 98.
Since the multiplying factor m is a negative value, the bandpass filter 102
selects the difference frequency component and outputs this component. As
a result, the target frequency F.sub.0 is output from the bandpass filter
102.
The frequency converter 82 may have the arrangement shown in FIG. 19.
In the frequency converter 82 having this arrangement, the first pilot
carrier signal f.sub.LO1 is multiplied by n (>0) by the multiplier 92.
In this case as well, the value n is constant independently of the
subcarrier center frequency of the intermediate frequency subcarrier
signal, unlike the arrangement in FIG. 18.
The second pilot carrier signal f.sub.LO2 is multiplied by
.vertline.m.vertline. by the multiplier 94. For the intermediate frequency
subcarrier signal f.sub.IF3, the multiplying factor m is -2, the second
pilot carrier signal f.sub.LO2 is multiplied by 2 by the multiplier 94.
The first pilot carrier signal f.sub.LO1 multiplied by n and second pilot
carrier signal f.sub.LO2 multiplied by .vertline.m.vertline. are supplied
to the mixer 96 and mixed. As a consequence, the sum frequency component
(n.times.f.sub.LO1 +.vertline.m.vertline..times.f.sub.LO2) and difference
frequency component (n.times.f.sub.LO1
-.vertline.m.vertline..times.f.sub.LO2) are output from the mixer 96.
As described above, since the value m is negative, of the sum frequency
component (n.times.f.sub.LO1 +.vertline.m.vertline..times.f.sub.LO2) and
difference frequency component (n.times.f.sub.LO1
-.vertline.m.vertline..times.f.sub.LO2) output from the mixer 96, the
difference frequency component is selected by the filter 100. The output
from the filter 100 is input to the mixer 98.
The mixer 98 mixes the intermediate frequency subcarrier signal f.sub.IF3
with the output from the filter 100. As a result, the sum frequency
component (n.times.f.sub.LO1 -.vertline.m.vertline..times.f.sub.LO2
+f.sub.IF3) and difference frequency component (n.times.f.sub.LO1
-.vertline.m.vertline..times.f.sub.LO2 -f.sub.IF3) between the two signals
is output from the mixer 98. This output signal is supplied to the filter
102.
Of the sum frequency component (n.times.f.sub.LO1
-.vertline.m.vertline..times.f.sub.LO2 +f.sub.IF3) and difference
frequency component (n.times.f.sub.LO1
-.vertline.m.vertline..times.f.sub.LO2 -f.sub.IF3) output from the mixer
98, the difference frequency component is selected by the filter 102.
Consequently, the target frequency F.sub.0 is output from the filter 102.
In the above examples, the value n is constant independently of the
subcarrier frequency in optical transmission. If the value n can change
depending on the transmitting/receiving device, the absolute values of n
and m can be decreased in accordance with a condition. This condition is
that the first pilot carrier signal f.sub.LO1 is an integral multiple of
the second pilot carrier signal f.sub.LO2 (=.DELTA.F). An example will be
described.
For example, an intermediate frequency subcarrier signal having a center
frequency f.sub.IF4 (=f.sub.IF1 +4.times..DELTA.F) when f.sub.LO1
=3.times.f.sub.LO2 will be considered.
Assume that the center frequency of a radio signal radiated from the
antenna 48 is F.sub.0 (=n.times.f.sub.LO1 +f.sub.IF1). In this case, as in
the above examples, the first pilot carrier signal f.sub.LO1 is multiplied
by n, a difference between the resultant signal and the second pilot
carrier signal f.sub.LO2 multiplied by 4 is calculated, and the
intermediate frequency subcarrier signal f.sub.IF4 is added to the
difference. That is, n.times.f.sub.LO1 -4.times.f.sub.LO2 +f.sub.IF4 is
calculated. With this frequency conversion, the target frequency F.sub.0
can be obtained.
Alternatively, since f.sub.LO1 =3.times.f.sub.LO2, the target frequency
F.sub.0 can be obtained by frequency conversion (n-1).times.f.sub.LO1
-f.sub.LO2 +f.sub.IF4. At this time, the first pilot carrier signal
f.sub.LO1 is multiplied by (n-1), and the second pilot carrier signal
f.sub.LO2 is multiplied by (.vertline.m.vertline.-3).
<Arrangement Using Transmitting/receiving Device with a Plurality of
Antennas>
The above description has been made in association with a system using a
transmitting/receiving device having one antenna. Each of the
transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p extracts one of
a plurality of intermediate frequency subcarrier signals
frequency-multiplexed and transmitted from the transmitting/receiving
station 10B.
However, an arrangement using a transmitting/receiving device with a
plurality of antennas may also be used. In this case, a plurality of
intermediate frequency subcarrier signals need be used by one
transmitting/receiving device.
FIG. 24 shows the arrangement of such a transmitting/receiving device 32C.
The transmitting/receiving device 32C comprises the O/E converter 34,
intermediate frequency subcarrier signal separation bandpass filters 38-1,
38-2, . . . , 38-N, the pilot carrier signal separation bandpass filters
40-1 and 40-2, the frequency converter 82 formed from a multiplier and a
power amplifier, and antennas 48-1, 48-2, . . . , 48-N. The bandpass
filters 38-1, . . . , 38-N are arranged in correspondence with the
antennas 48-1, . . . , 48-N, respectively. The bandpass filters 38-1, . .
. , 38-N extract intermediate frequency subcarrier signals for
corresponding antennas 48-1, . . . , 48-N from an electrical signal from
the O/E converter 34 and are formed from simple filters having small Q
value.
In this arrangement, an optical signal transmitted from the
transmitting/receiving station 10B as shown in FIG. 15 is
photoelectrically converted into an electrical signal. From the electrical
signal obtained by photoelectrically converting a received optical signal
by the O/E converter 34, the first pilot carrier signal separator 40-1
extracts the first pilot carrier signal f.sub.LO1, and the second pilot
carrier signal separator 40-2 extracts the second pilot carrier signal
f.sub.LO2.
When the transmitting/receiving device 32C uses N (N.gtoreq.2) intermediate
frequency subcarrier signals, the intermediate frequency subcarrier
signals are separated by the bandpass filters 38-1, 38-2, . . . , 38-N
formed from simple filters.
The separated intermediate frequency subcarrier signals are converted into
frequencies to be radiated from the antennas 48-1, 48-2, . . . , 48-N by
the frequency converter 82. The signals are sent to the antennas 48-1,
48-2, . . . , 48-N and radiated into air.
For the frequency converter 82, the arrangement shown in FIG. 18, 19, or 22
may be prepared in number corresponding to the number of intermediate
frequency subcarrier signals. Alternatively, portions where the
frequencies of the pilot carrier signals f.sub.LO1 and f.sub.LO2 are
multiplied by a desired value and added, which are common to the
intermediate frequency subcarrier signals, may be shared by the
intermediate frequency subcarrier signals.
In this embodiment, if the value N is large, the transmitting/receiving
stations 10B and transmitting/receiving devices 32C may be connected in a
one-to-one correspondence instead of connecting one transmitting/receiving
station to a plurality of transmitting/receiving devices as in the above
examples.
The above description has been made about the transmission system (down
link signal processing system). A reception system is also necessary, and
the reception system (up link signal processing system) of this system
will be described next as a ninth embodiment.
Ninth Embodiment
The ninth embodiment is associated with a reception system (up link signal
processing system). When three or more intermediate frequency subcarrier
signals, i.e., intermediate frequency subcarrier signals of three or more
systems are to be received by different antennas, only two pilot carrier
signals f.sub.LO1 and f.sub.LO2 are used for frequency conversion of the
intermediate frequency subcarrier signals, an intermediate frequency
subcarrier signal of a system is converted into a signal with a desired
frequency using the pilot carrier signals and the intermediate frequency
subcarrier signal of that system and optically transmitted to a
transmitting/receiving station.
FIG. 25 is a block diagram showing an embodiment of the reception system,
i.e., up link signal processing system of such a system. As shown in FIG.
25, as the system arrangement of the reception system, a receiving station
10D incorporates a data separation demodulator 140 and an O/E converter
138.
Each of a plurality of receiving devices 32D-1, 32D-2, . . . , 32D-p has an
E/O converter 136, a frequency converter 134, and an antenna 132. The
frequency converter 134 receives the first and second pilot carrier
signals f.sub.LO1 and f.sub.LO2.
The plurality of receiving devices 32D-1, 32D-2, . . . , 32D-p and
receiving station 10D are connected through optical fibers 30. Optical
signals received by the receiving devices 32D-1, 32D-2, . . . , 32D-p are
coupled by an optical divider 34 inserted midway along the optical fibers
30, and are guided to the receiving station 10D.
The frequency converter 134 frequency-converts a received radio signal into
an intermediate frequency subcarrier signal f.sub.IF using the first and
second pilot carrier signals f.sub.LO1 and f.sub.LO2 and the carrier
component of the radio signal. The first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2 are transmitted from a device with exception of
the receiving device 32D-1. It is most reasonable to use the pilot carrier
signals f.sub.LO1 and f.sub.LO2 from the terminal station 10D, which are
separated in the transmission system (down link signal processing system).
The E/O converter 136 converts the intermediate frequency subcarrier signal
f.sub.IF frequency-converted by the frequency converter 134 into an
optical signal and optically subcarrier-transmits the optical signal to
the optical fiber 30.
The O/E converter 138 in the receiving station 10D converts the optical
signal optically subcarrier-transmitted through the optical fiber 30 into
an electrical signal. The data separation demodulator 140 separates the
electrical signal converted by the O/E converter 138 into intermediate
frequency subcarrier signals in units of channels and demodulates the
signals.
In the system having this arrangement, the radio signals having center
frequencies F.sub.0, which are received by the antennas 132 of the
receiving devices 32D-1, . . . , 32D-p, are frequency-converted into
intermediate frequency subcarrier signals f.sub.IF1, . . . , f.sub.IFp by
the frequency converters 134. The first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2 are input to each frequency converter 134. Signals
obtained by appropriately multiplying the two pilot carrier signals
f.sub.LO1 and f.sub.LO2 are appropriately added/subtracted to/from the
frequencies of the radio signals having the center frequencies F.sub.0 to
obtain the intermediate frequency subcarrier signals f.sub.IF1, . . . ,
f.sub.IFp. The signals converted into the intermediate frequency
subcarrier signals f.sub.IF1, . . . , f.sub.IFp are converted into optical
signals by the E/O converters 136 and optically subcarrier-transmitted to
the receiving station 10D through the optical fibers 30.
FIG. 26 is a graph showing the frequency arrangement of the intermediate
frequency subcarrier signals f.sub.IF1, . . . , f.sub.IFp. The frequency
converters 134 of the receiving devices 32D-1, . . . , 32D-p use different
multiplying factors for the two pilot carrier signals f.sub.LO1 and
f.sub.LO2 and different signs for addition/subtraction.
Optical signals output from the plurality of receiving devices 32D-1, . . .
, 32D-p are coupled by the optical divider 34 and converted into an
electrical signal by the O/E converter 138 in the receiving station 10D.
For the spectrum of the electrical signal received and converted by the
O/E converter 138, the center frequencies have a difference such that the
signal can be separated in units of devices, as shown in FIG. 26. At the
demodulator 140, the received signal is separated to each subcarrier
signal corresponding with each intermediate frequency band and demodulated
to the data.
With this arrangement, each different subcarrier frequency which is
converted data signals in the same radio frequency band into at the
receiving devices becomes stable.
Like the down link signal (transmission signal) processing system shown in
FIG. 24, the up link signal (reception signal) processing system may also
employ an arrangement using one transmitting/receiving device having a
plurality of antennas. In this arrangement, intermediate frequency
subcarrier signals received by the plurality of antennas are converted
into different frequencies and optically transmitted to the
transmitting/receiving station.
FIG. 27 shows the arrangement of a receiving device 32E. Referring to FIG.
27, antennas 150-1, . . . , 150-N are antennas of different systems.
Outputs from the antennas 150-1, . . . , 150-N are transmitted to the
optical fiber 30 through a frequency converter 152, a mixer 154, and an
E/O converter 156. The frequency converter 152 converts the radio signals
received by the plurality of antennas 150-1, . . . , 150-N into different
subcarrier frequencies using the first and second pilot carrier signals
f.sub.LO1 and f.sub.LO2.
The mixer 154 mixes the subcarrier signals of different antenna systems,
which are output from the frequency converter 152. The E/O converter 156
modulates the mixed subcarrier signal into an optical signal and outputs
the optical signal to the optical fiber 30.
In the system having the above arrangement, radio signals are received by
the plurality of antennas 150-1, . . . , 150-N of the receiving device
32E. The received radio signals are converted into different subcarrier
frequencies by the frequency converter 152 using the first and second
pilot carrier signals f.sub.LO1 and f.sub.LO2.
The pilot carrier signals f.sub.LO1 and f.sub.LO2 are transmitted from an
external device of the receiving device 32E. The frequency converter 152
appropriately multiplies and adds/subtracts the pilot carrier signals
f.sub.LO1 and f.sub.LO2 to frequency-convert the radio signals received by
the antennas.
The multiplying factors and, in some cases, the signs for
addition/subtraction of the pilot carrier signals f.sub.LO1 and f.sub.LO2
change in units of signals received by the antennas.
The frequency converter 152 may have independent multiplication and
addition/subtraction means in correspondence with the antennas, or a
common portion may be shared.
The signals frequency-converted into different subcarrier frequencies by
the frequency converter 152 are added by the adder 154, converted into an
optical signal by the E/O converter 156, and transmitted to the receiving
station. As in FIG. 25, signals from the plurality of receiving devices
may be mixed and transmitted to the receiving station. Alternatively, the
receiving devices and receiving stations may be connected in a one-to-one
correspondence.
In the above-described examples, the transmitting/receiving device has only
the down link signal processing system or only the up link signal
processing system. However, a transmitting/receiving device incorporating
both a down link signal processing system and up link signal processing
system is also necessary. An example, will be described below.
Tenth Embodiment
An example of a transmitting/receiving device incorporating a down link
signal processing system and up link signal processing system will be
described. The communication system to be described below has the
characteristic features of both the above-described arrangement applied to
a down link signal and that applied to an up link signal. The
transmitting/receiving device also uses two pilot carrier signals
f.sub.LO1 and f.sub.LO2 contained in an optical signal sent from a
transmitting/receiving station for frequency conversion of an up link
signal received by the antenna.
FIGS. 28 and 29 show arrangements of the transmitting/receiving device. The
transmitting/receiving device shown in FIG. 28 has both the
transmitting/receiving device structures shown in FIGS. 15 and 25. In
addition, the pilot carrier signals f.sub.LO1 and f.sub.LO2 obtained by
the system for processing a down link signal are divided and input to the
system for processing an up link signal.
More specifically, the down link signal processing system of a
transmitting/receiving device 32F comprises the O/E converter 34, bandpass
filter 38 for extracting an intermediate frequency subcarrier signal,
bandpass filters 40-1 and 40-2 for extracting pilot carrier signals,
frequency converter 82, and antenna 48. The up link signal processing
system comprises an antenna 132, a frequency converter 134, and an E/O
converter 136.
The bandpass filters 40-1 and 40-2 extract the first and second pilot
carrier signals f.sub.LO1 and f.sub.LO2. The pilot carrier signals
f.sub.LO1 and f.sub.LO2 are used by the frequency converter 82 in the down
link signal processing system and also supplied to the frequency converter
134 in the up link signal processing system. The frequency converter 134
frequency-converts a received radio signal into an intermediate frequency
subcarrier signal f.sub.IF using the first and second pilot carrier
signals f.sub.LO1 and f.sub.LO2.
As described above, the first and second pilot carrier signals f.sub.LO1
and f.sub.LO2 extracted by the bandpass filters 40-1 and 40-2 in the down
link signal processing system are used not only by the frequency converter
82 in the down link signal processing system but also by the frequency
converter 134 in the up link signal processing system to frequency-convert
an up link signal.
In the arrangement shown in FIG. 28, the transmitted pilot carrier signals
f.sub.LO1 and f.sub.LO2 are separated and independently multiplied and
added/subtracted in the frequency converters 82 and 134. After this,
frequency conversion is performed to obtain a predetermined frequency. A
portion capable of partially sharing the multiplication and
addition/subtraction functions may be shared. An example is shown in FIG.
29.
In FIG. 29, a frequency converter 160 which integrates the frequency
converters 82 and 134 of the arrangement shown in FIG. 28 is used. In this
frequency converter 160, the multiplication and addition/subtraction
functions are partially shared by up and down link signals.
In the frequency converter 160, a multiplier output and, as needed, a mixer
output of the frequency converter for a down link signal in FIG. 18 or 19
are divided and used for frequency conversion of an up link signal.
In addition, as shown in FIG. 30, outputs from the bandpass filters 40-1
and 40-2 are input to a local carrier generation unit 166. The local
carrier generation unit 166 generates local carrier signals to be used to
frequency-convert up link and down link signals.
A frequency converter 168 for an up link signal and a frequency converter
170 for a down link signal only add/subtract the local carrier signals
generated by the local carrier generation unit 166 to/from the frequencies
of signals before frequency conversion. Hence, each of the frequency
converters 168 and 170 has a simple arrangement mainly comprising a mixer
and a filter.
In the above example, the antennas for up link and down link signals are
separated. However, when one antenna can be used for both up link and down
link signals, a circulator is connected to one antenna such that the up
link system and down link system can share the antenna through the
circulator.
In this form, in a transmitting/receiving station 10I, local carrier
signals can be used to generate subcarrier signals for a down link signal
and also to frequency-convert subcarrier signals for an up link signal
before modulation. FIG. 31 is a block diagram showing an example of the
transmitting/receiving station 10I with such an arrangement.
<Arrangement of Subcarrier Signal Sharing Transmitting/Receiving
Station>
The transmitting/receiving station 10I shown in FIG. 31 has, as a down link
system (transmission system), the E/O converter 18 formed from the laser
element 86 and the laser driver 84, adder 16, frequency converters 88-1,
88-2, . . . , 88-p each comprising a local carrier generator 172 and a
mixer 174, modulators 12-1, 12-2, . . . , 12-p, and first and second pilot
carrier generators 14-1 and 14-2.
The modulators 12-1, 12-2, . . . , 12-p modulate input data and output them
to the corresponding frequency converters 88-1, 88-2, . . . , 88-p,
respectively.
Local carrier generators 172-1, . . . , 172-p generate different local
carrier signals. The local carrier generators 172-1, . . . , 172-p are
arranged, respectively, corresponding to frequency converters 88-1, 88-2,
. . . , 88-p and output local carrier signals to corresponding mixers
174-1, 174-2, . . . , 174-p, respectively. Each of the mixers 174-1,
174-2, . . . , 174-p converts the modulated input signal into the
intermediate frequency subcarrier signal f.sub.IF having a desired center
frequency using the frequency of the input signal and local carrier signal
and outputs the converted signal.
The first and second pilot carrier generators 14-1 and 14-2 generate the
first and second pilot carrier signals f.sub.LO1 and f.sub.LO2 having
different frequencies, respectively. The adder 16 synthesizes the two
pilot carrier signals f.sub.LO1 and f.sub.LO2 and outputs from the
frequency converters 88-1, 88-2, . . . , 88-p. The laser driver 84 drives
the laser element 86 in accordance with the signal synthesized by the
adder 16. The laser element 86 outputs a laser beam optically modulated in
accordance with the synthesized signal from the adder 16 and sends the
laser beam to the optical fiber 30.
The reception system, i.e., the up link signal processing system comprises
the O/E converter 138, intermediate frequency subcarrier signal separators
178-1, . . . , 178-p, mixers 180-1, . . . , 180-p for frequency
conversion, and demodulators 182-1, . . . , 182-p.
The O/E converter 138 is connected to an optical divider (not shown) and
receives an optical signal transmitted from the transmitting/receiving
device side through the optical fiber 30 and converts the optical signal
into an electrical signal. Each of the intermediate frequency subcarrier
signal separators 178-1, . . . , 178-p separates and extracts an
intermediate frequency subcarrier signal for a specific one of the
plurality of transmitting/receiving devices from the electrical signal
from the O/E converter 138 and is formed from a simple filter or the like.
The mixers 180-1, . . . , 180-p are arranged, respectively, corresponding
to intermediate frequency subcarrier signal separators 178-1, . . . ,
178-p. Each of the mixers 180-1, . . . , 180-p receives a local carrier
signal from a corresponding one of the local carrier generators 172-1, . .
. , 172-p and frequency-converts an intermediate frequency subcarrier
signal using the intermediate frequency subcarrier signal obtained from a
corresponding one of the intermediate frequency subcarrier signal
separators 178-1, . . . , 178-p.
The demodulators 182-1, . . . , 182-p are arranged, respectively,
corresponding to mixers 180-1, . . . , 180-p. Each of the demodulators
182-1, . . . , 182-p demodulates the intermediate frequency subcarrier
signal frequency-converted and supplied from a corresponding one of the
mixers 180-1, . . . , 180-p and outputs the demodulated signal.
In this arrangement, data signals from transmitting/receiving devices are
modulated by, e.g., QSPK by the corresponding modulators 12-1, . . . ,
12-p.
The signals modulated by the modulators 12-1, . . . , 12-p are
frequency-converted into intermediate frequency subcarrier signals for
optical transmission by the corresponding frequency converters 88-1, . . .
, 88-p, respectively. More specifically, each of the frequency converters
88-1, . . . , 88-p adds/subtracts the frequency of the local carrier
signal generated by a corresponding one of the local carrier generators
172-1, . . . , 172-p to/from the input band signal, thereby performing
frequency conversion.
The first and second pilot carrier generators 14-1 and 14-2 generate the
pilot carrier signals f.sub.LO1 and f.sub.LO2 having different
frequencies, respectively. The adder 16 synthesizes the two pilot carrier
signals f.sub.LO1 and f.sub.LO2 and outputs from the frequency converters
88-1, 88-2, . . . , 88-p, and outputs the synthesized signal to the laser
driver 84. The laser driver 84 drives the laser element 86 in accordance
with the signal synthesized by the adder 16 to generate a laser beam
optically modulated in accordance with the synthesized signal from the
adder 16, and sends the laser beam to the optical fiber 30.
In the up link signal processing system, an optical signal transmitted
through the optical fiber 30 is converted into an electrical signal by the
O/E converter 138 and supplied to the intermediate frequency subcarrier
signal separators 178-1, . . . , 178-p.
The intermediate frequency subcarrier signal separators 178-1, . . . ,
178-p separate intermediate frequency subcarrier signals of predetermined
channels from the electrical signal. Each of the mixers 180-1, . . . ,
180-p converts the intermediate frequency subcarrier signal supplied from
a corresponding one of the intermediate frequency subcarrier signal
separators 178-1, . . . , 178-p into a signal having a predetermined
center frequency using the local carrier signal.
More specifically, each of the mixers 180-1, . . . , 180-p receives a local
carrier signal from a corresponding one of the local carrier generators
172-1, . . . , 172-p. The intermediate frequency subcarrier signal is
converted into a specific center frequency using the local carrier signal
and the carrier component of the intermediate frequency subcarrier signal
obtained from a corresponding one of the intermediate frequency subcarrier
signal separators 178-1, . . . , 178-p. The specific center frequency is
the same as the output frequency from the modulators 12-1, 12-2, . . . ,
12-p.
The intermediate frequency subcarrier signals frequency-converted by the
mixers 180-1, . . . , 180-p are demodulated by the corresponding
demodulators 182-1, . . . , 182-p, respectively.
In this embodiment, in the arrangement having the down link signal
processing system and up link signal processing system in the
transmitting/receiving station 10I, local carrier signals are shared by
the down link signal processing system and up link signal processing
system.
More specifically, a local carrier signal is input from the local carrier
generator 172-1 to the mixer 174-1 in the frequency converter 88-1,
another local carrier signal is input from the local carrier generator
172-2 to the mixer 174-2 in the frequency converter 88-2, and still
another local carrier signal is input from the local carrier generator
172-p to the mixer 174-p in the frequency converter 88-p. In this manner,
local carrier signals are supplied from local carrier generators of the
corresponding systems and also supplied to the mixers 180-1, . . . , 180-p
in the up link signal processing system. More specifically, a local
carrier signal is input from the local carrier generator 172-1 to the
mixer 180-1, another local carrier signal is input from the local carrier
generator 172-2 to the mixer 180-2, and still another local carrier signal
is input from the local carrier generator 172-p to the mixer 180-p.
In the present invention, up link subcarrier signals transmitted from the
transmitting/receiving device having the arrangement as shown in FIG. 28,
29, or 30 are multiplexed at a sufficiently large frequency interval, as
shown in FIG. 5, like the down link subcarrier signals. The frequency
interval is the same as that between the down link subcarrier signals.
In the transmitting/receiving station 10I, the output from the local
carrier generator 172-1 is divided into two paths. One path is connected
to the mixer 174-1 in the frequency converter 88-1, and the other path is
connected to the mixer 180-1. The up link intermediate frequency
subcarrier signals f.sub.IF1, . . . , f.sub.IFp obtained by converting an
optical signal into an electrical signal and separating the signal by the
intermediate frequency subcarrier signal separators 178-1, . . . , 178-p
are converted by the mixers 180-1, . . . , 180-p, into frequencies
suitable for demodulation by the demodulators 182-1, . . . , 182-p,
respectively.
Since the local carriers from the local carrier generators 172-1, . . . ,
172-p are used by the mixers 180-1, . . . , 180-p, the frequencies of
signals to be input to the demodulators 182-1, . . . , 182-p can be easily
and accurately controlled.
With this arrangement, the subcarrier frequencies between the
transmitting/receiving devices when up link signals are
subcarrier-multiplexed can be easily stabilized, and the frequencies of
demodulator inputs of up link signals in the transmitting/receiving
station are stabilized, so frequency control for demodulation is
facilitated. Additionally, since the local carrier generator can be shared
in the transmitting/receiving station, the equipment can be reduced.
Furthermore, since local carriers are generated in the
transmitting/receiving device and used for frequency conversion of both of
the up link and down link signals, the frequency conversion unit of the
transmitting/receiving device can be simplified.
The conversion frequency interval for frequency conversion of up link and
down link signals in the transmitting/receiving devices may be different
between the up link and down link. When down link signals are
subcarrier-multiplexed as shown in FIG. 5, the up link signals need not be
arranged in the same order as that of the down link signals, i.e., in the
order of transmitting/receiving devices 32B-1, 32B-2, . . . , 32B-p in
ascending order of frequencies. The order may be changed by appropriately
changing the conversion frequency interval of the up link signals.
When the radio signal frequency radiated from the antenna largely changes
between the up link and down link, the conversion frequency interval of up
link signals is intentionally made different from that of down link
signals in the transmitting/receiving devices such that the frequencies
are in almost the same frequency band in optical subcarrier transmission.
With this arrangement, the cost can be reduced because the subcarrier
frequency band which allows inexpensive optical subcarrier transmission is
limited.
In the above examples, the transmitting/receiving device has antennas
independently prepared for the up link signal system and down link signal
system. However, as in the arrangement for the up link signal system or
down link signal system (FIG. 24 or 27), a plurality of antennas for the
up link signal system or down link signal system may be arranged.
The above embodiments have been described about only an optical fiber
network called a PON. However the present invention can be applied to
another form such as a cable coaxial transmission or HFIFC (Hybrid fiber
Coax) in which a signal is transmitted through an optical fiber and then
divided through a coaxial cable.
The present invention is not limited to the above-described embodiments,
and various changes and modifications can be made. For example, the
multiplying factor of the frequency multiplier for multiplying a pilot
carrier signal is not limited to an integral multiple and may be a decimal
multiple.
As has been described above, according to the present invention, even when
a large optical modulation index is set for the pilot carrier signal
f.sub.LO, the CNR of the intermediate frequency subcarrier signal f.sub.IF
does not decrease, and the pilot carrier signal f.sub.LO with a high CNR
can be obtained on the transmitting device side. Since a radio frequency
signal received by the antenna of the receiving device may be weak, a
signal for frequency conversion by the multiplier is required to have a
high CNR. As the signal for frequency conversion, the pilot carrier signal
f.sub.LO can be provided from the transmitting station. When the pilot
carrier signal f.sub.LO is multiplied as the signal for frequency
conversion, the noise characteristics are not largely degraded in
frequency conversion because the CNR of the received pilot carrier signal
f.sub.LO is high.
By adding the pilot carrier signal f.sub.LO, an increase in RIN value of
the intermediate frequency subcarrier signal f.sub.IF band can be
suppressed. Hence, degradation in CNR of the intermediate frequency
subcarrier signal f.sub.IF band can be reduced, and the optical modulation
index of the pilot carrier signal f.sub.LO to be transmitted to the
transmitting device side can be made large without increasing the RIN of
the intermediate frequency subcarrier signal f.sub.IF band. Since the CNR
of the pilot carrier signal f.sub.LO received on the transmitting device
side can be made high, a high-quality radio frequency signal can be
obtained while suppressing additive noise in frequency conversion. Since
any degradation in CNR characteristics of the intermediate frequency
subcarrier signal f.sub.IF and pilot carrier signal f.sub.LO can be
suppressed, the optical fiber transmission distance can be increased. For
example, when optical analog transmission of the present invention is
applied to a radio communication base station, the communication service
area covered by one transmitting station can be expanded.
According to the present invention, when one transmitting/receiving station
accommodates a plurality of transmitting/receiving devices through a PON,
the frequency stability between the transmitting/receiving devices can be
maintained using a simpler optical transmission system. More specifically,
when data signals subcarrier-multiplexed are to be distributed from a
transmitting/receiving station to a plurality of transmitting/receiving
devices, the intermediate frequency subcarrier signals to be used by the
transmitting/receiving devices are subcarrier-multiplexed at a
sufficiently large frequency interval such that the intermediate frequency
subcarrier signals can be separated by a simple filter after reception of
an optical signal. In addition, the radio frequency is set such that only
two pilot carrier signals suffice to synchronize the frequencies of radio
waves radiated from the transmitting/receiving devices (independently of
the number of transmitting/receiving devices).
As a consequence, a communication system in which while establishing
frequency synchronization between the transmitting/receiving devices,
satisfactory transmission can be performed without sacrificing the optical
modulation index of the intermediate frequency subcarrier signal in
optical subcarrier transmission due to transmission of the pilot carrier
signal, and the process of extracting necessary signals after reception of
an optical signal is easy and inexpensive can be provided.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the present invention in its broader aspects is not
limited to the specific details, representative devices, and illustrated
examples shown and described herein. Accordingly, various modifications
may be made without departing from the spirit or scope of the general
inventive concept as defined by the appended claims and their equivalents.
* * * * *
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